Popular Science Monthly/Volume 41/September 1892/Odors and the Sense of Smell
By M. CHARLES HENRY.
A CONSIDERABLE number of mineral compounds are odorous. It is enough to mention, as illustrations of the fact, the hydrogen odor of rotten eggs, and the scent of hydrocyanic acid which emanates from bitter almonds. Although perfumes, or pleasant smells, are organic or carbon compounds, the distinction between organic and inorganic may be considered artificial, since the principal organic bodies can be obtained by the combination of such simple mineral elements as carbon, oxygen, hydrogen, and nitrogen. On the gradual complication of syntheses of this kind M. Berthelot, who has made more of them than any other chemist, has based a classification of organic compounds into eight categories. We have first, hydrocarbons, formed of the two elements—acetylene, formene, benzene, turpentine, styrolene, etc. The bodies composed of three elements—carbon, hydrogen, and oxygen—are divided among four categories. We distinguish between the alcohols, which are capable of uniting directly with acids to form ethers with the elimination of the elements of water; the aldehydes, which are formed at the expense of the alcohols, with the loss of hydrogen, among which are the essence of bitter almonds and the essence of cinnamon; the acids, like acetic and benzoic acids, which, can unite with bases and form salts; and the ethers, the results of the association of alcohols, acids, or other alcohols, among which are the oils of the onion and of mustard. Among quaternary compounds we have the alkaloids formed by the union of the alcohols with ammonia or other alkalies, amides formed by the union of ammonia and acids with the separation of the elements of water; and the metallic radical compounds which are obtained by the reaction of metals on some of the ethers.
Perfumes are, in general, binary or ternary compounds characterized by the fact that the proportion of equivalents of hydrogen to those of carbon diminishes at the same rate as those of another class of products very rich in hydrogen which are called the fatty series, while this class of products, less rich in hydrogen, is called the aromatic series. Is there any relation between odor and chemical composition? An English physiologist, Mr. John Berry Hay craft, in his studies of the savors and odors, and savors of the principal compounds of each natural family of bodies, particularly of compounds of the family oxygen, sulphur, chromium, selenium, molybdenum, tellurium, didymium, tungsten, and uranium, has observed modifications in odor corresponding with increase in atomic weights. For example, sulphureted hydrogen, hydrogen selenide, and hydrogen telluride smell like rotten eggs. The compounds of elements of this family with methyl and ethyl have an alliaceous odor. So with the family chlorine, bromine, and iodine; the acids which these bodies form with hydrogen and their compounds with methyl, ethyl, and ethylene have similar odors, so that some among them seem to share them with their neighbors; bromof orm, for example, having a similar odor with chloroform and iodoform. Passing to the organic series, Mr. Haycraft observes in the monatomic alcohols a modification of odor corresponding with variations in atomic weight. Methyl alcohol, for instance, has a weak odor of alcohol; ethyl alcohol has the typical alcoholic odor; propylic alcohol has both an alcoholic odor and a special smell; isobutylic, amylic, and octylic alcohols progressively lose the alcoholic odor and acquire as against it a special scent. The same facts are remarked in the fatty acids and hydrocarbons.
Similar odors may be furnished by bodies without likeness in chemical composition. Arsenic in oxidizing disengages vapors that have the odor of garlic. Nitrobenzene, benzoic aldehyde, and prussic acid smell much alike. It has been asserted that emeralds pounded and ground several hours a day for three weeks had emitted a well-defined odor of violets. The fact has been verified; but it has yet to be determined whether it is due to the manipulation or to organic substances that have been released by the trituration. Sulphuric acid, cortibined with distilled water, disengages a pungent odor resembling that of musk. The odor of musk is brought out in a great many reactions. The nitrate derivatives of aromatic substances smell of it; artificial musk and natural musk have no chemical resemblance. So alcohols chemically identical, but of different derivation, do not behave alike with essential oils. As odor is thus in a great measure independent of the chemical constitution, it must depend upon the disposition of the particles, a property which it is evidently impossible to discover by any known chemical processes.
A few eminent chemists, following Dalton, Avogadro, and Ampère, have tried to make up for this impossibility by hypothesis, and have taken up the great problem of predicting and explaining chemical combinations and isomerics. Their theories, called atomic, have been adopted in most of the original memoirs and taught in most of the text-books. Whatever may be their scientific value, the aids they give him in retaining and recollecting the formulas present incontestable advantages to the student. The applications of them to the study of the aromatic series are famous.
The radical of the hydrocarbons of this series and of all the other compounds is benzene, a body composed of six atoms of carbon and six atoms of hydrogen; when it is attacked by a reagent, and we substitute for an atom of hydrogen another simple body or a group of atoms, whichever of the atoms of hydrogen the substitution may bear upon, we obtain a single product; whence it is concluded that each atom of carbon is united to an atom of hydrogen, and that a symmetrical exchange can take place of the atoms of carbon among their valencies. A German chemist, Herr Kekulé, has tried to express these peculiarities by a hexagonal scheme which has still some lack of symmetry, and M. Ladenbourg has substituted a prismatic scheme for it. In this figure the six atoms of carbon of the benzene occupy the summits of a triangular prism, each one being united with an atom of hydrogen and exchanging the three valencies that are left it with the three next atoms of carbon by the three edges which meet at the summit. The perfect symmetry of this scheme is well expressed in the simple construction of the figure. But usually, for greater convenience, the hexagonal construction is adopted, and the reciprocal relations of the atoms of carbon and hydrogen are represented by figures in which the more or less complex lateral chains are joined, and which offer the remarkable characteristic of being closed chains—that is, of always returning to their starting-point. What the atomic theories have taught us concerning odor is limited to this singular and so far unfruitful representation; it is evident that they are still mute concerning the real structure of the molecular edifice. The efforts which have been recently made to fill this void are more difficult to expound and follow than fruitful in applications.
Six methods of extracting perfumes are known: The first is expression, by means of a special press, which is applicable without too great loss to fruit-skins rich in essential oils, such as orange and citron peel, previously grated. Another method is that of distillation, which consists in heating flowers with water in a boiler. The essential oil is volatilized and is condensed with the vapor of water in a worm and a Florentine receiver. The water usually goes to the bottom and the oil floats. The oils of neroli, rose, patchouli, geranium, lavender, caraway, etc., are obtained in this way. This process is not applicable to the delicate perfumes of the mignonette and the violet; and for them recourse is had to maceration of the flowers in animal fats or mineral oils, which have the property of absorbing odorous substances, and are then washed in alcohol. The flowers are usually heated in the fat or the oil for a variable number of hours. For perfumes which can not endure a high temperature the petals are placed between two frames of glass coated with fat. This is the process of enfleurage. The pneumatic process, which consists in causing a current of perfumed air or carbonic acid to be absorbed by coatings of lard on glass plates, appears not to have given satisfactory results. Another process consists in dissolving perfumes in very volatile liquids like sulphuret of carbon, chloroform, naphtha, ether, or chloride of methyl, and volatilizing the solvents, which can be done at a low temperature in a vacuum. The last method has given very satisfactory results in the extreme delicacy and great accuracy of its returns.
Numerous classifications of odors have been proposed. It is, of course, impossible to quote any rational classification. The natural way is to group around a type, in successive series, odors which resemble one another. Eugene Rimmer has tried to do this in the accompanying table.
The author observes that it would be hard to arrange in any of these series certain peculiar odors like that of wintergreen, or salicylate of methyl and magnolia. Notwithstanding the uncertainties attending the arrangement, we must apparently depend upon classifications based upon this principle for a guide in the study of odors.
All that we know concerning the propagation of an odor is that it consists in an emission of solid, liquid, or gaseous particles. This emission is allied for these three states of matter to the property called diffusion, which consists in the reciprocal penetration at the end of a certain time of the particles of two or more bodies among one another; and also for solids and liquids to the property called volatility, or the rapidity of evaporation.
But little is known concerning the diffusion of solids. If we heat to a high temperature a porcelain crucible within a crucible of plumbago, the plumbago will penetrate the porcelain to a depth varying according to the duration of the experiment. M. Pellat has shown, by delicate measures of quantities of electricity, that metallic surfaces placed parallel to one another a few tenths of a millimetre apart, reciprocally exchange their outer surfaces, as if they emitted a little of their own substance to each other. When the influence ceases, the surfaces gradually lose their foreign coatings, and return slowly to their primary condition.
The diffusion of liquids is easily observed. It can be witnessed by introducing, with a pipette, into a vessel under water a colored liquid, red wine, for example. The wine, being lighter than water, rises to the surface, and does not color the deeper layers of the water till after one or two days. There is doubtless in the complicated diffusion of liquids a kind of chemical action related to the movements on water of camphor and a considerable number of diffusible substances. If we put a bit of camphor on the surface of water, it at once turns round and moves in every direction. If a drop of oil is let fall on the same surface, the movements will cease immediately. The motion arises from the diffusion of camphor in a liquid form on the surface of water. When, after the surface is saturated, there is no more diffusion, the motions cease. They also cease when two currents are produced by different bodies in opposite directions. That there is a liquid diffusion is proved by the fact that when the camphor is placed on a float of pith, or on the polished surface of mercury, there is no movement. So, if a bit of camphor is put into a large saucer covered with a thin layer of water, the water immediately retires, sometimes for several centimetres, before the odorous substance. The laws of the diffusion of liquids may be summarized by saying that the rapidity depends on the nature of the substance, increases in proportion to the degree of concentration of the solution, and augments as the temperature rises. Graham's dialyzer is based on the very feeble diffusibility of certain substances, like the gums, and the great diffusibility of certain crystalline substances, like salt. It is simply a vessel, the bottom of which is formed of a leaf of parchment paper, that lets the diffusible substances pass into the water around it and holds the others.
The diffusion of gases and vapors, which is more important in questions of smell, is subject to laws which have been only approximately determined. A glass tube about a metre long is used, divided perpendicularly to its length by a thin metallic partition, which can be made to slide between two perforated glasses. A gas is introduced into each of the separated halves of the tube; the supply-cocks are closed, the partition is lifted out, and the two halves of the tube are put in communication; a half-hour later the partition is shut, and the gaseous mixture contained in each of the compartments is analyzed. Mr. Loschmidt has in this way found the mathematical rule for the measure of the diffusion of different gases, one within the other.
The volatility of a liquid is expressed by the weight of that liquid which evaporates per second and per square millimetre at a given temperature. All that is known of it is that this weight is proportioned to the excess of the maximum tension of the vapor at that temperature over the tension which it has in the air; and this weight varies inversely as the atmospheric pressure according to a law special for each liquid. Evaporation may, therefore, give us valuable information concerning the purity of the odor, and spare us, in many cases, the delicate problem of determining the maximum tension which is so important a characteristic of substances. A special apparatus has been devised for the rapid measurement of volatility.
Tables have been prepared showing the relative volatility of different perfumes, of the substances used for adulterating them, and of the adulterations, by means of which a convenient method is afforded for the detection of frauds.
The influence of different physical forces on the disengagement of odor has been studied; and possible relations between the colors of flowers and the intensity of their perfumes have been inquired into. It has been found that white flowers represent the largest number of odoriferous species, and after them come red, yellow, green, and blue. The order corresponds with that of the emission of calorific force. Flowers which by their color emit the most heat, also emit the most perfume.
The results of the study of the influence of the color of substances on their power of absorbing odors differ a little from these: white, yellow, red, green, and blue absorb odors in a decreasing order, or rather emit them in an increasing one. These colors represent decreasing luminous powers.
Ozone develops the energy of essential oils, and perfumes in turn determine by their oxidation in the air the production of ozone. This is a matter of hygienic significance, for the presence of ozone being favorable to health, we have a means at hand of increasing the supply of it by surrounding ourselves with fragrant substances and flowers.
Heat favors the volatilization of perfumes, and to such an extent that beds of flowers are sometimes inodorous in the bright sunlight which are fragrant in the shade. Some essences need a high temperature for the production of their full effect; while others, to have their delicacy fully appreciated, require the coolness of the evening. This principle may account for appir-fnt differences of tastes among the people of different countries. The odors of many substances are not of equal strength in different climates. Prof. Tyndall believes that there are considerable differences in the absorbing power of different odorous vapors for radiant heat. He perfumed small paper cylinders by dipping them by one end in an aromatic oil, and then placed them in a glass tube, which communicated, through a stop-cock, with a tube in which a vacuum is produced. The air, according as it has been perfumed with one substance or another, discloses to the galvanometer an absorbing power, which, air at the usual pressure being taken as one, varies from thirty for patchouli, to three hundred and seventy-two for anise-seed. These results are, unfortunately, not exact, for no account is taken in them of the tensions of the odorous vapors, which certainly vary, though they are probably of very small absolute value.
Messrs. Nichols and Bailey have compared the smelling powers of men and women. Having made measured solutions of a number of essential oils, a series of flasks was prepared so that the solution in each succeeding one should be only half as strong as that in the preceding one. The flasks were "shuffled," and the subjects of the experiment were called upon to rearrange them in the order of concentration of the solutions. The smelling power of women appeared to be on the whole less delicate than that of the men. The extreme delicacy of the scent of the dog is well known. Mr. Romanes has shown that, by fastening a sheet of paper to the shoes, the odor may be masked, and the dog prevented from following the track of his master; but that a contact with the ground of a few square millimetres is enough to enable the dog to follow the scent. In birds, the sense of smell appears to be little developed; in mollusks and insects the smelling apparatus has been located in the antennæ. Below the group of worms, no olfactory reactions have been, so far as I know, definitely established.
The mechanism of the olfactory apparatus is, as a whole, simpler than that of sight and hearing; but the sensation is subordinated to many individual anatomical peculiarities. As much can be said of touch and taste, which require contact of the excitant, while sight and hearing merely register the vibrations transmitted by a medium. It is easy to conceive how the condition of the membranes, the form of the nasal passages, etc., may affect the sensation.
A distinction is made in medicine between respiratory anosmias which depend on the formation of the organs and the condition of the connective tissues, and essential anosmias which result from atrophy of the nerves. Anosmias are frequent; some are congenital, many are senile and temporary, and connected with traumatisms, hemianesthesia, aphasia, and hemiplegia. We can not expect to find as concordant reactions for the smell as for the sense of color or the sense of form. It is nevertheless a matter of interest to investigate, on as good subjects as we can get, the influence of different odors on sensibility; or, in other words, to determine the weight of odorous vapor which it is necessary to breathe and accumulate in the nasal f ossse to make a perfume perceptible. That is the purpose of olfactometers. The olfactometer gives, besides this, the intensity of a perfume. The larger the perceptible minimum of a perfume, the less intense the perfume is, and it is this intensity which determines the price of a perfume, the delicacy of its odor being the same.
The olfactory sense is followed by effects of different kinds of intensity from those of sight and hearing, and may be accompanied by a kind of poisoning. The old medical books are full of stories of it. There are those of a girl killed by the exhalations of violets; of a woman seized with a violent headache from sleeping on a bed of roses; and of a girl who lost her voice by smelling of a bouquet. Ancient medicine attributed curative properties to perfumes, particularly to those of the rose, musk, and benzoin. The intensity of the effects of perfumes makes a rapid succession of sensations almost impossible; for consecutive odors cause a rapid anæsthesia of the sense; on the other hand, if the times separating two successive sensations are too long, it becomes impossible to combine them, and the anticipated effect is disturbed by strange feelings. In short, smell is rather the complement of other excitations than an artistic excitation like a melody or a picture. Its function is, nevertheless, very important. By virtue of its volatility it is a valuable prophylactic; by the great intensity of its effects it can bring about salutary modifications of physiological functions, particularly of the amplitude of respiration; and it possesses in the highest degree the luxurious character of every artistic enjoyment. Flavor has an essential part in nutrition; so has touch. Hearing and sight are indispensable to relations with other persons; but smell, necessary to the animal for finding its prey and avoiding danger, has become, under normal conditions, an almost useless sense to man, since the refinements of civilization tend to prevent the production of miasms and the pestilential odors from which he has to protect himself. It is therefore becoming more and more a sense of luxury for civilized man; and that, perhaps, is the reason why poets, from the author of the Song of Songs down, have associated all kinds of beauty and joy with perfumes.—Translated for The Popular Science Monthly from the Revue Scientifique.