Popular Science Monthly/Volume 12/February 1878/The Hygienic Influence of Plants

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THE HYGIENIC INFLUENCE OF PLANTS.
By MAX VON PETTENKOFER.

THE animal kingdom is, as we know, dependent on the vegetable kingdom, which must have existed on the earth before men and animals could live upon it. We may, therefore, rightly call plants children of the earth. But, in so doing we use the language of metaphor, as when we speak of "Mother Earth." The earth does not directly bring forth either plants or animals. Every plant is the child of a mother-plant, descends from one of its own kind like ourselves; but plants derive their nourishment directly from earth, air, and water, and, although generated by plants, are nourished directly by the inorganic breasts of Nature, and imply no other organic life but their own. Had plants a voice, they would more correctly speak of "Mother Earth" than ourselves.

Plants live directly on the lifeless products of earth, and we live directly on the products of plants or on animals which live on them; our existence implies other organic life, and our nourishment is not derived so directly from the earth as that of plants. Since the vegetable world comes between us, we should rather call earth our grandmother than our mother. At all events it is an affectionate relationship.

We have a natural feeling of close affinity with the vegetable world, which expresses itself not only in our love of foliage and flowers, but in our fondness for metaphors derived from the vegetable world and its processes. If we were to reckon up how many metaphors in every-day life and in poetry are derived from the vegetable world, and how many from other spheres of Nature, we should find a great excess of the former.

Our material relations to plants are also very numerous. The question we are now concerned with is not what food or what medicinal remedies plants provide us with, but the value of plants and plantations in dwellings and in the open air in conducing to health or preventing disease. We have given the subject very little consideration until quite recently, just as we have thought very little of the way in which the pleasures of the table, fine raiment, comfortable dwellings, and many other things, conduce to our well-being. Meanwhile we have been guided by our instincts, which, like Nature in general, have, on the whole, guided us rightly. Even now there is not much scientific knowledge on the subject; still there is a little, and something is gained when we begin seriously to reflect on anything, for knowledge is sure then to increase. All that man has ever aspired to and attained has always existed much earlier in idea than in reality. Ideas are never fully realized, as we all know, and it is only very gradually that they are realized at all.

It is generally asserted that vegetation purifies the air, and chiefly by three functions: firstly, because plants absorb carbonic acid; secondly, because under the influence of sunlight they exhale an equivalent in oxygen; and, lastly, because they produce ozone. These facts I need not demonstrate, as they have been placed beyond doubt by vegetable physiologists, chemists, and meteorologists. My task is to show what the direct sanitary effect of these three functions is.

I must at once state that none whatever can be proved to exist. And, as this assertion will contradict the prepossessions of many readers, I feel bound to prove my proposition.

As to carbonic acid, the first question is: What is the proper and normal proportion of this gas in the air, next how much more carbonic acid is contained in air which is notoriously bad; and, lastly, whether the air on a surface without vegetation contains essentially more carbonic acid than one having vegetation upon it?

The amount of carbonic acid in the open air has been often determined, and is confined within very narrow limits. It may be said—leaving severe storms or very thick fogs out of the question—to vary between three and four parts in each 10,000 of the volume of the air.

Experiments have also been made on the quantity of carbonic acid in apartments occupied by man, and it is generally taken as the criterion of the quality of the air, ventilation being regulated by it. In very bad air which is undoubtedly deleterious, it has been found to amount to from three to five per mille. One per mille marks the boundary-line between good and bad air in a room.

We next inquire whether the atmosphere over a vast tract of country destitute of vegetation contains more carbonic acid than one abounding in vegetation, whether in the former case the amount of carbonic acid approaches one per mille. In 1830 De Saussure began to make researches into the variations in the quantity of carbonic acid in Geneva, and they were continued about ten years later by Verver in Holland, and Boussingault in Paris; in more recent, and very recent times, a great number of experiments have been made on the subject by Roscoe in Manchester, Schulze at Rostock, and myself and my pupils, particularly Dr. Wolffhügel, at Munich. The result is, in the main, that the variations—very small from the first—have been found to be still smaller as the methods of determining carbonic acid have been perfected.

Saussure, who worked by a method liable to give an excess, found from 3.7 to 6.2 parts in 10,000. He considered that there were also slight variations between summer and winter, day and night, town and country, land and sea, mountains and valleys, which might be ascribed to vegetation. Boussingault, however, found the carbonic acid in the air to be rather less, and the same on an average in Paris and St.-Cloud: in Paris 4.13 and at St.-Cloud 4.14 in 10,000, which surprised him the more as he had reckoned that in Paris at least 2,944,000,000 litres of carbonic acid were exhaled by men, animals, and fuel.

Roscoe made experiments on the air at a station in the middle of Manchester, and at two stations in the country. He was originally of opinion that the vast manufactures of Manchester, chiefly dependent on the consumption of coal, must produce a perceptible effect on the carbonic acid in the air; but he also discovered that the air in the space in front of Owens College contained no more than the air at the country stations. He also observed occasional variations: but, when the carbonic acid increased or diminished in the city, it was generally just the same in the country. Roscoe found the greatest amount of carbonic acid in the air during one of the thick fogs prevalent in England.

Schulze found the amount of carbonic acid in the air at Rostock to be between two and half and four parts in 10,000. On an average it was somewhat higher when the wind blew off-shore than off the sea.

In Munich, Wolffhügel found the carbonic acid to be between three and four parts in 10,000. Now and then, but very seldom, he observed variations, the maximum being 6.9 parts in 10,000 in a very thick fog, the minimum 1.5 part in 10,000 in a heavy snow-storm, when the mercury was very low in the barometer.

It may be asked how the immense production of carbonic acid in a city like Paris or Manchester can thus vanish in the air. The answer is very simple: by rarefaction in the currents of the atmosphere. We are apt not to take this factor into account, but think rather of the air as stagnant. The average velocity of the air with us is three metres per second, and even in apparently absolute calm it is more than half a metre. If we therefore assume a column of air 100 feet high and of average velocity, it may be reckoned that the carbonic acid from all the lungs and chimneys of Paris or Manchester is not sufficient to increase its amount so as to be detected by our methods.

From this fact it may be logically concluded that, if no increase in the carbonic acid in the air is observable, no diminution will be observable from vegetation.

It is a universally recognized and incontrovertible fact that the carbonic acid contained in all the vegetable life on earth is derived from the carbonic acid in the air, in water, and the soil. Many conclude, therefore, that the air in a green wood must contain less carbonic acid than that in a city or that of an extensive tract of waste land. But I can assure them that the air in the Sahara, so called, of Munich, formerly called the Dultplatz, contains no more carbonic acid than the neighboring Eschen-grounds. Of this I can give incontestable proof, an argument ad hominem. Dr. Zittel brought me several specimens of air in hermetically-sealed glass tubes, from his travels in the Libyan Desert, from sandy wastes and from oases, on which I could conveniently make experiments at Munich. The amount of carbonic acid does not differ in the least in the air from the barren waste and the greenest oases. The case is just the same with the amount of oxygen in the air. It was formerly thought, when imperfect methods were employed, that perceptible variations could be proved. Thus, for example, the outbreak of cholera in 1831 was attributed to a diminution of oxygen in the air, and here and there experiments were made which seemed to confirm the opinion. The hypothesis did not seem improbable, for it was concluded with certainty that in tropical swamps, which are the home of cholera, the oxygen in the air might have been in course of time diminished by the vast masses of decaying matter. But, since the method of gas analysis has been arranged by Von Bunsen, the amount of oxygen in the air on the summit of Mont Blanc has not been found to differ from that in a city or in the swamps of Bengal. Neither is it greater in forest or sea air than in the air of the desert.

This absence of demonstrable variation, in spite of the production of oxygen by living plants and the absorption of it by the processes of combustion and decay, becomes intelligible when we consider first the mobility, and then the mass of the air encompassing our earth. The weight of this mass is, as the barometer tells us, equal to that of a layer of mercury which would cover the surface of the earth to the depth of 760 millimetres (more than three-quarters of a metre). From the weight of this, several billion kilometres, some idea can be formed of the volume of the air, when we consider that air, even beneath a pressure of 760 millimetres of mercury, is yet 10,395 times lighter than mercury. In masses like these, variations such as those we speak of go for nothing. The amount of carbonic acid and oxygen might perhaps be essentially changed in Paris or Manchester if all organic matter on and in the earth were burning at once.

Even if it is granted, however, in face of these incontrovertible facts, that vegetation exercises no perceptible influence upon the composition of the atmosphere in the open air, many persons will not be disposed to give up the idea that the air in rooms can be improved by plants, because, as is well known, every green leaf absorbs carbonic acid and gives out oxygen under the influence of light. This idea may seem the more justifiable, because, although the production of carbonic acid is not perceptible in the greatest assemblages of human beings in the open air, it is always observed in confined spaces, although the actual production is but small. In the air of a closed apartment, every person and every light burning makes a perceptible difference in the increase of carbonic acid in the air. Must not, therefore, every plant in a pot, every spray, any plant with leaves, make a perceptible difference in a room? Every lover of flowers may be pardoned for wishing to see this question answered in the affirmative. Have not even medical men proposed to adorn school-rooms with plants in pots instead of ventilating them better, in order that their leaves and stems might absorb carbonic acid from the mouths of the children, and give out oxygen in its stead? But hygiene cannot agree even to this. Hygiene is a science of economics, and every such science has to ask not only what exists and whether it exists, but how much there is and whether enough. The power of twenty pots of plants would not be nearly sufficient to neutralize the carbonic acid exhaled by a single child in a given time. If children were dependent on the oxygen given off by flowers, they would soon be suffocated. It must not be forgotten what a slow process the production of matter by plants is—matter which the animal organism absorbs and again decomposes in a very short time, whereby as much oxygen is used up as has been set free in the production of it. It is for this reason that such great extents of vegetation are required for the sustenance of animals and man. The grass or hay consumed by a cow in a cow-house grows upon a space of ground on which a thousand head of cattle could stand. How slow is the process of the growth of wheat before it can be eaten as bread, which a man will eat, digest, and decompose, in twenty-four hours! The animal and human organism consumes and decomposes food as quickly as a stove burns the wood which took so many thousand times longer to grow in the forest.

It would scarcely be intelligible if I were to calculate how much carbonic acid and oxygen a rose, a geranium, or a bignonia, would absorb and give out in a room in a day, and to what extent the air might be changed by it, taking into account the inevitable change of air always going on. I will draw attention to a concrete case which every one can understand:

When the Royal Winter Garden in Munich was completed and in use, it occurred to me to make experiments on the effect of the whole garden on the air within it. There could not be a more favorable opportunity for experimenting on the air in a space full of vegetation. This green and blooming space was not exposed to the free currents of air which at once immensely rarefy all gaseous exhalations, but was kept warm under a dome of glass, through which only the light of heaven penetrated. Although not hermetically sealed, the circulation of air in such a building, compared with that in the open air, is reduced over a hundred-thousandfold.

I asked permission to make experiments for several days at various hours of the day and night, which was readily granted. Now, what was the result? The proportion of carbonic acid in the air in the winter garden was almost as high as in the open air. This greatly surprised me, but I hoped at any rate to have one of my traditional ideas confirmed: I hoped to find less carbonic acid in the day than in the night, supported by the fact that the green portions of plants under the influence of light decompose carbonic acid and develop oxygen. But even here I was disappointed. I generally found carbonic acid increasing from morning till evening, and decreasing from night till morning. As this seemed really paradoxical, I doubled my tests and care, but the result remained the same. At that time I knew nothing of the large amount of carbonic acid of the air, in the soil, the air of the ground, or I should probably have been less surprised.

One day it suddenly became clear to me why there was always more carbonic acid by day than by night. I had been thinking only of the turf, the shrubs, and trees, which consume carbonic acid and produce oxygen, and not of the men and birds in the winter garden. One day, when there were considerably more men at work there than usual, the carbonic acid rose to the highest point, and sank again to the average during the night. The production of carbonic acid by the working and breathing of human beings was so much greater than that consumed by the plants in the same time.

The oxygen in the winter garden was rather higher than in the open air; there it was about twenty-one per cent., and in the winter garden twenty-two to twenty-three per cent.

I did not make any experiments on ozone, for reasons which I will give by-and-by.

The amount of carbonic acid in the air in the winter garden cannot be reckoned as telling for or against the hygienic value of vegetation in an inclosed space. Let us inquire, then, into the value of the slight increase of oxygen.

There is a wide-spread opinion that the breathing of air rich in oxygen effects a more rapid transformation of matter, a more rapid combustion, as we say, in the body. Even great inquirers and thinkers have considered that we only eat and imbibe nourishment to satiate the oxygen streaming through us, which would otherwise consume us. We know now well enough that the quantity of oxygen which we imbibe does not depend on the quantity in the air we breathe, but far more on previous changes in and the amount of transformation of matter in the body, which are regulated by the requirements of breathing. The inhalation of oxygen is not a primary but a secondary thing. When we inhale air at every breath richer than usual in oxygen—for example, when breathing highly compressed air, as divers do, or laborers on the pneumatic foundations of bridge-piers—the result is not a larger consumption of matter and an increased production of carbonic acid, but merely a decrease in the number of inhalations. If in air of ordinary density we make about sixteen respirations in a minute, in air of greater density we should involuntarily make only twelve, ten, or eight, according to the density and our need of oxygen; all else remains the same.

Lavoisier, and, half a century later, Regnault and Reiset, placed animals for twenty-four hours in air very rich in oxygen, but they did not consume more of it than in the ordinary air. An increase of oxygen in the air, therefore, or pure oxygen gas, only produces an effect in certain morbid conditions, in cases of difficulty of breathing, or where breathing has been for some time suspended, because an inspiration communicates more oxygen to the blood than breathing ordinary air. A healthy person can, however, without difficulty or injury, compensate for considerable differences, and an increase or decrease of one or two per cent, of oxygen does no harm, for under ordinary circumstances we only inhale one-fourth of the oxygen in the air we breathe; we inhale it with twenty-one per cent, and exhale it with sixteen per cent.

So far, therefore, as we feel ill or well in a winter garden, it does not depend on the quantity of oxygen in the air, and there is no greater appreciable quantity of oxygen in a wood of thick foliage than in a desert or on the open sea.

Let us also for a moment consider the ozone in the air, which may be looked upon as polarized or agitated oxygen. After its discovery, which has immortalized the name of Schönbein, was made known, it was thought for a time that the key had been found for the appearance and disappearance of various diseases, in the quantity of ozone in the air. But one fact, which was observed from the first, shows that it cannot be so; for the presence of ozone can never be detected in our dwellings, not even in the cleanest and best ventilated. Now, as it is a fact-that we spend the greater part of our lives in our houses, and are better than if we lived in the open air, the hygienic value of ozone does not seem so very great. Added to this, the medical men of Königsberg long had several ozone-stations there, during which time various diseases came and went, without, as appears from the reports of Dr. Schiefferdecker, ozone having the slightest connection with the appearance or disappearance of any of them.

Dr. Wolfhügel, assistant at the Hygienic Institute at Munich, has lately been occupied with the question of the sanitary value of ozone, but has arrived at only negative results.

But in saying this I have no intention of denying that ozone is of great importance in the atmosphere, for I am of opinion that it is. It is the constant purifier of the atmosphere from all organic matter, which passes into it and might accumulate. The air would have been long ago filled with the vapors of decomposition if it were not for ozone, which oxidizes all that is oxidizable, if only time enough is allowed for it, and too much is not expected at once; for, generally, the amount of ozone in the air is so small that it is consumed in making its way into our houses, without disinfecting them, and we can no more dispense with the greatest cleanliness and best ventilation in our homes than we can essentially change the air in our rooms by means of plants in pots and foliage.

Some of my readers will perhaps ask in some disappointment, "In what, then, does the hygienic value of plants and plantations consist? Or do I mean to say that all the money spent by one and another on a parterre of flowers in his house or on a garden, or by a community for beautiful grounds, or by a state for the preservation of forests, with the idea of promoting health, is mere luxury, without any hygienic value?" These questions alter our standpoint, and I believe I shall be able to show that even hygiene does recognize a sanitary value in plants and flowers, in the laying out of grounds and plantations, only it offers a different explanation from the ordinary one.

I consider the impression which plants and plantations make upon our minds and senses to be of hygienic value; further, their influence on the conformation of the soil, with which health is in many respects connected; and, finally, their influence upon other qualities of the air, than carbonic acid, oxygen, and ozone: among these may be mentioned, in passing, shade in summer, and decrease of wind and dust.

It is an old observation, needing no demonstration, that the cheerful and happy man lives not only an easier, but, on the average, a more healthy life than the depressed and morose man. Medical men, and especially "mad doctors," could tell us much of the great value of a certain relative proportion of pleasurable and painful impressions upon health, and how frequently some unfortunate position, an absence of pleasure, or too much of painful impression, is the cause of serious illness. Man always tries, and has an irresistible need, to balance painful sensations by some kind of pleasure or other, so that often, in order to get himself into a tolerable frame of mind, or to deaden his feelings for a time, he will have recourse to wine, beer, or spirits, though he knows well enough that he will be worse afterward than before. A certain amount of change and recreation is indispensable, and, failing others, we seek them by injurious means. There are, doubless, some unhappy and morbid natures who are always discontented, to whom everything comes amiss, and whom it is impossible to help; but the majority of men are easily pleased, find pleasure in little things, though it is but a sorry life they lead. It is something the same with the pleasures of life as with the pleasures of the table; we must relish our food if it is to do us good. What good will the most nourishing diet do me if it creates disgust? Prof. C. Voit has clearly pointed out, in his experimental researches into diet, the great value of palatable food, as well as nourishment, and how indispensable a certain variety in our meals is. We think we are only tickling the palate, and that it is nothing to the stomach and intestines whether food is agreeable to the palate or not, since they will digest it, if it is digestible at all. But it is not so indifferent, after all; for the nerves of the tongue are connected with other nerves and with the nerve-centres, so that the pleasures of the palate, or some pleasure, at any rate, even if it is only imagination, which can only originate in the central organ, the brain, often has an active effect on other organs. This is a matter of daily experience. If you put your finger down your throat, you produce retching; many people have only to think of anything disgusting to produce the effect of an emetic, just as the thought of something nice makes the mouth water just as much as tasting the most dainty morsel. Voit showed me one of his dogs with a fistula in the stomach. So long as this dog is not thinking of food, his stomach secretes no gastric juice, but no sooner does he catch sight of a bit of meat, even at a distance, than the stomach prepares for digestion and secretes gastric juice in abundance. Without this secretion the assimilation of nourishment would be impossible. If, therefore, some provocatives induce and increase certain sensations and useful processes, they are of essential value to health, and it is no bad economy to spend something on them.

I consider flowers in a room, for all to whom they give pleasure, to be one of the enjoyments of life, like condiments in food. It is certainly one of the most harmless and refined. We cannot live on pleasure alone; but, to those who have something to put up with in life, their beloved flowers perform good service.

The same may be said of private gardens and public grounds, and of the artistic perfecting of them. The more tastefully laid out, the better the effect. Though tastes differ, there is a general standard of taste which lasts for several generations, though it varies from time to time, and is subject to fashion. As their object is to give pleasure, public grounds should accord with the taste of the age, or aim at cultivating it. This is a justification for going to some expense for aesthetic ends.

The influence of vegetation on the soil is much more easy to determine than on the mind of man. Space fails me to go into all the aspects of this subject, and I will confine myself to some of the most obvious. The difference is most apparent on comparing the soil of a tract of land covered with wood with the soil outside, in other respects alike. The Bavarian Forest Department deserves great credit for having established meteorological stations with special reference to forest-culture, under the superintendence of Prof. Ebermayer, of Aschaffenburg. He has published his first year's observations in a work on "The Influence of Forests on the Air and Soil, and their Climatic and Hygienic Importance,"[1] which may be recommended to every one who wishes to study the subject.

Modern hygiene has observed that certain variations in the moisture of the soil have a great influence on the origin and spread of certain epidemic diseases, as for instance cholera and typhoid fever— that these diseases do not become epidemic when the moisture in the soil is not above or below a certain level, and has remained so for a time. These variations can be measured with greater accuracy by the ground-water of the soil than by the rainfall, because in the latter case we have to determine how much water penetrates the ground, how much runs off the surface, and how much evaporates at once. The amount of moisture in the soil of a forest is subject to considerably less variation than that outside. Ebermayer has deduced the following result from his meteorological observations on forestry: "If from the soil of an open space 100 parts of water evaporate, then from the soil of a forest free from underwood 38 parts would evaporate, and from a soil covered with underwood only 15 parts would evaporate." This simple fact explains clearly why the cutting down of wood over tracts of country is always followed by the drying up of wells and springs.

In India, the home of cholera, much importance has been attached in recent times to plantations as preventives of it. It has been always observed that the villages in wooded districts suffer less than those in treeless plains. Many instances of this are given in the reports of Dr. Bryden, President of the Statistical Office in Calcutta, and Dr. Murray, Inspector of Hospitals. For instance, Bryden[2] compares the district of the Mahanadda, one of the northern tributaries of the Ganges, the almost treeless district of Rajpoor, with the forest district of Sambalpoor. It is stated that in the villages in the plain of Rajpoor, sixty or seventy per cent, of the inhabitants are sometimes swept away by cholera in three or four days, while the wooded district of Sambalpoor is often free from it, or it is much less severe. The district commissioner, who had to make a tour in the district on account of the occurrence of cholera, reports, among other things, as follows:

"The road to Sambalpoor runs for sixty or seventy miles through the forest, which round Petorah and Jenkfluss is very dense. Now, it is a remarkable fact, but it is a fact, nevertheless, that on this route, traversed daily by hundreds of travelers, vehicles, and baggage-trains, the cholera rarely appears in this extent of sixty miles, and when it does appear it is in a mild form; but when we come to the road from Arang, westward to Chicholee Bungalow, which runs for about ninety miles through a barren, treeless plain, we find the cholera every year in its more severe form, the dead and dying lying by the wayside, and trains of vehicles half of whose conductors are dead."

In the same report Dr. Bryden continues:

"I will mention one other fact as a result of my observations, namely, that places surrounded by those vast and splendid groves which are occasionally seen, lying in low and probably marshy situations, surrounded by hills, and which, from the mass of decaying vegetation, are very subject to fever in September, October, and November, are seldom visited by cholera, and if it occurs there are but few deaths, while places on high ground, or in what are called fine, airy situations, free from trees and without hills near, so that they are thoroughly ventilated, suffer very much from cholera."

Murray gives a number of instances showing the influence of trees on the spread of cholera. One of these may find a place here:

"The fact is generally believed, and not long ago the medical officer of Jatisgar, in Central India, offered a striking proof of it. During the wide-spread epidemic of cholera in Allahabad, in 1859, those parts of the garrison whose barracks had the advantage of having trees near them enjoyed an indisputable exemption, and precisely in proportion to the thickness and nearness of the shelter. Thus the European Cavalry in the Wellington Barracks, which stand between four rows of mango trees, but are yet to a certain extent open, suffered much less than the Fourth European Regiment, whose quarters were on a hill exposed to the full force of the wind; while the Bengal Horse Artillery, who were in a thicket of mango-trees, had not a single case of sickness; and the exemption cannot be regarded as accidental, as the next year the comparative immunity was precisely the same."[3]

We need not, however, go to India to observe similiar instances of the influence of a certain degree of moisture in the soil favored by woods or other conditions; we can find them much nearer home. In the cholera epidemic of 1854, in Bavaria, it was generally observed that the places in the moors were spared, in spite of the otherwise bad condition of the inhabitants. The great plain of the Danube from Neuburg to Ingolstadt was surrounded by places where it was epidemic, while in the plain itself there were but a few scattered cases. The same thing has been demonstrated by Reinhard, President of the Saxon Medical College. Cholera has visited Saxony eight times since 1836, and every time it spared the northerly district between Pleisse and Spree, where ague is endemic.

In the English Garden at Munich there are several buildings, not sparsely tenanted—the Diana Baths, the Chinese Tower, with a tavern and out-buildings, the Gendarmerie Station, and the Kleinkessellohe. In the three outbreaks of cholera at Munich none of these places have been affected by it. This fact is the more surprising, as three of them comprise public taverns into which the disease germs must have been occasionally introduced by the public; yet there was no epidemic in these houses, although it prevailed largely immediately beyond the English Garden and close to the Diana Baths in 1854 and 1873. It must have been accidental that no isolated cases occurred, as the inmates of the Chinese Tower, or the Kleinkessellohe, might have caught it in Munich as others did who came from a distance, but, had there been single cases, probably no epidemic would have occurred in these houses.

Even if these deductions must be accepted with caution from an etiological point of view, still, on the whole, they indisputably tell in favor of trees and woods.

Surface vegetation has also other advantages, besides its use in regulating the moisture in the soil; it purifies it from the drainage of human habitations, whereby it is contaminated and impregnated. If this refuse matter remains in soil destitute of growing vegetation, further decomposition sets in, and other processes are induced, not always of a salubrious nature, but often deleterious, the products of which reach us by means of air or water, and may penetrate into our houses. But from this indisputable fact false conclusions are sometimes drawn. Many people imagine that if a few old trees are left standing in an open space their roots will absorb all the impurities from the houses around, and render the refuse which accumulates beneath them innocuous. This idea is not only false in a sanitary point of view, but very injurious, as it prevents people from taking the measures which alone can keep the ground under our houses pure.

We will now explain why the shade of gardens and woods is at certain seasons so beneficial. The human race during its pilgrimage on earth and wanderings over it has many difficult tasks to perform. One of the most difficult is involved in the necessity that all our internal organs, and the blood, whether at the equator or the north pole, should retain an equable temperature of 3712° Centigrade (98° Fahr.). Deviations of but one degree are signs of serious illness. The blood of the negro and that of the Esquimaux is of the same temperature, while the one lives in a temperature of 40° above, and the other 40° below zero (Centigrade). A difference of 80° has therefore to be equalized.

Our organism, doubtless, possesses a special apparatus for the performance of this colossal task, self-acting sluices so to speak, by means of which more or less of the heat generated in the body passes off: these consist mainly in the increase or diminution of the peripheric circulation, and the action of the pores of the skin. But we soon come to the end of our natural regulating apparatus, and have to resort to artificial means. Against cold we have excellent methods in clothing, dwellings, and fires; but at present our precautions against heat are very limited. This is, doubtless, the reason why higher civilization has extended so much farther toward the polar regions than toward the equator. The Germanic races, particularly, inevitably degenerate after living for a few generations in the tropics, and must be continually renewed by immigration if they desire to retain supremacy, as is proved by the case of the English in India. They will not be able to settle there and maintain the characteristics which have made them dominant, until means have been found of diminishing the heat of the body at pleasure, as we are able to maintain it in the north. At present our remedies against heat are baths, fans, and shade.

We lose the heat of our bodies in three different ways: by the medium in which we are, generally the air, and which can be warmed; by the evaporation of perspiration; and by radiation from bodies of a lower temperature, not taking into account a small portion of heat which goes off in mechanical labor. Under ordinary circumstances in temperate climates, we lose half the heat generated by radiation, one-fourth by evaporation, and one-fourth by the conducting medium in which we are. In proportion as any of these methods is diminished, one or both the others must be increased. As long as possible, our organisms are so obliging as to open and close the sluices themselves without our cognizance, provided that our regulating apparatus is in order, that we are not ill. It is only when our good servant the skin, under certain conditions, has come to an end of its powers, that we begin to feel that we must lend our aid. And thus we have found by experience that, in hot weather, shade helps the body to keep cool to the needful extent. The chief effect of shelter is to prevent the sun's rays from striking us directly; but, if this were all, it would be as cool in the height of summer in-doors, or even under the leaden roofs of Venice, which have driven many to frenzy and desperation, as under the shade of a tree or in a wood. It also makes a great difference whether the sun's rays fall on thick foliage or on a roof of slate or metal. A great deal of heat is neutralized by evaporation from the leaves; another portion by the decomposition of carbonic acid, just so much as is set free when we burn the wood and other organic combinations into the composition of which it enters. The heat produced by burning wood in a stove is derived from the sun; it is but the captured rays of the sun again set free by combustion. We learn from Ebermayer's work that the temperature of the trees in a forest, and even in the tops of them, is always lower than the air in the forest.

Besides this, shade in the open air always causes a certain draught which acts as a kind of fan. All must have noticed when walking in oppressive heat, when the air seems still as death, that a refreshing breeze arises as soon as a cloud casts a shade. The same thing may often be observed in summer in walking through a street with close rows of houses, when the air is still, and one side is sunny, the other in shade. On the sunny side there is not a breath of air, while on the other there may be a light breeze. This is easily explained; so far as the shade extends the air is cooler than in the sun; layers of air of unequal warmth are of different gravity, and this difference of temperature is the cause of the motion in the air.

The shade of a single tree, therefore, cools not only by intercepting the sun's rays, but also by the effect of gentle fanning. The shelter of a thick wood, however, is much more agreeable than that of a single tree. The air in a wood is cooler than that of an open space exposed to the sun. The air from outside is drawn into the wood, is cooled by it, and cools us again. And it is not only the air that cools us, but the trees themselves. Observation has shown that the trunks of trees in a wood breast-high, even at the hottest time of day, are 5° Centigrade cooler than the air. We therefore lose considerable heat by radiation to these cooler objects, and can cool ourselves more easily at a temperature of 25° Centigrade in a wood than at a much lower temperature in an open space. When the objects around us are as warm as ourselves we lose nothing by radiation; what is radiated from us is radiated back by them. This is why we are so uncomfortable in heated and overcrowded rooms. It is generally set down to bad air, and this does certainly contribute to it, but it is chiefly the result of disturbed distribution of heat, as has been plainly shown by experiments on the composition of such air, which makes many people feel ill.—Contemporary Review.

 
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  1. "Die physikalischen Wirkungen des Waldes auf Luft und Boden und seine klimatologische und hygienische Bedeutung."
  2. "Epidemic Cholera in the Bengal Presidency," 1869, p. 225.
  3. "Report on the Treatment of Epidemic Cholera," 1869, p. 4.