Popular Science Monthly/Volume 5/October 1874/Thermal Death-Point of Living Matter I
WATER is boiling merrily over a brisk fire, when some luckless person upsets the vessel, so that the heated fluid exercises its scathing influence upon an uncovered portion of the body—hand, arm, or face. Those who have seen much of the effects produced upon the human skin by such accidents, will have acquired information not unworthy of influencing their opinion on some more general problems connected with the action of heat upon living matter. Here, at all events, there is no room for doubt. Boiling water unquestionably exercises a most pernicious and rapidly destructive action upon the living matter of which we are composed. There is no need to appeal to the sufferer's sensations for this information. This, indeed, is a point of view which we may for the present dismiss. For, however agonizing these sensations may be, they could only supply us with information upon a collateral point with which we are not at present concerned. Apart from such subjective effects, there are objective effects. That is, we are easily able to see the changes produced by boiling water upon living matter—revealing themselves as they do by an immediately altered appearance of the skin, and by the terrible wound so quickly produced. Upon these distressing, though, unfortunately, only too familiar consequences of the action of heat upon living matter, it is not necessary for me further to dwell; I would merely have the reader so far bear them in mind that they may not be incapable of recall during the perusal of this article. The occasional revival of such impressions may perhaps prove a little instructive to some who may chance to he at all dubious as to the destructive effects of boiling water upon lower organisms.
Probably, however, some of my readers may already be possessed by the notion that the disastrous effects just referred to are consequences following rather from the fact of the high organization of man's tissues than from any intrinsic incompatibility of nature between living matter and boiling water. The thought is natural enough and not unjustifiable. On the other hand, it will not do to attach much importance to it. Let us for a moment consider the effects produced upon an ordinary hen's egg by a brief immersion in boiling water. Here we have the "white," composed of albumen, similar to that which enters so largely into the composition of living tissues, turned from a clear fluid into an opaque solid; and we have the "yelk," made up of a dense aggregation of the simplest living units, also more or less solidified. In spite of the investing calcareous shell, these very obvious and destructive effects can be produced upon this large egg or germ by an exposure for three or four minutes to the influence of boiling water. Yet the living matter in this case is so simple that it possesses next to no organization—it is so little vitalized that it can only be considered to be half alive.
The conclusion would seem, therefore, to force itself upon us that there is something intrinsically deleterious in the action of boiling water upon living matter—whether this living matter be of high or of low organization.
This subject is one of great importance in many respects, so that it may repay us to look into the evidence bearing upon it with some degree of care. It is of great practical importance, for instance, in reference to the process of disinfection by heat, where we have to do with articles of furniture or wearing apparel used by a person suffering from a contagious disease. Because, in such a case, what we ought undoubtedly to know is, whether the temperature of boiling water, or even some lower temperature, suffices to kill any living particles which may act as so-called "germs of disease." This is a subject upon which there should be no room for doubt. Again, from a purely scientific point of view, the question is of equal cogency because of its bearing upon one of the most momentous problems in biological science—namely, that of the Origin of Life. It is on this latter account, more especially, that I now take up the inquiry as to the grade or degree of heat which proves destructive to different kinds of living matter.
A preliminary word of explanation, therefore, must be given concerning the bearings of this question upon the Origin of Life problem.
It is at present very generally admitted, upon the strength of well-known experiments, that living matter will appear and grow rapidly in hermetically-sealed flasks containing certain fluids, after the flasks and their contents have been thoroughly raised to the temperature of boiling water for ten minutes or more. These experiments we may mentally label as series A. Other experiments, which we may similarly label series B, had also shown that a brief exposure in the moist state to a temperature considerably below the boiling-point of water, is destructive to all kinds of living matter submitted to its influence. The experiments of series A, therefore, taken in conjunction with those of series B, must (if the latter results are as reliable as the former) be held to prove that living matter can originate independently, or de novo, through the mere productive properties of certain infusions or solutions.
If the facts are true, is it possible to stave off the conclusion? While the candid reader is asking himself this question, I may further point out to him that, as the previously discredited results belonging to series A are no longer denied, doubt is now only possible upon a subject hitherto supposed to be settled—namely, as to whether living matter is really killed by exposure in the moist state to a temperature of 212° Fahr. Obviously, at such a juncture, it rested more especially with the panspermatists, who chose still to be opponents of "spontaneous generation," to show that this belief concerning the destructive efficacy of boiling water, upon the truth of which they had previously relied, was erroneous—seeing that the advocates of spontaneous generation had demonstrated the truth of their position with reference to experiments A. Should the panspermatists fail to produce this evidence as to the untruthfulness of their old view, they must not expect to hear that they have the best of the argument; and still less will they be able to hold their ground if, while abstaining themselves from all experiments belonging to series B, their scientific opponents do make careful investigations in this direction, and arrive at the conclusion that not only was the old opinion right as to the destructive action of boiling water, but that living matter unaccustomed to the influence of heat is killed by a brief exposure even to the much lower temperature of 140° Fahr.
This is the present aspect of the problem, and those most interested in it may remember that knowledge would not advance in the rapid way which it does, were it not for the fact that the difficulties of one generation of men often disappear before the clearer, because more unprejudiced, vision of the next. Growing gradually more familiarized with the facts, those who come after us will be more and more influenced by them, and at the same time less warped by theoretical considerations already out of harmony with our present state of knowledge. We are now in a stage of transition. We are gradually learning to accept the doctrines of Evolution, as applicable to different departments of knowledge, though, as is so frequently the case when new doctrines are being adopted, this transition is being effected by many in a partial manner—they, unconsciously perhaps, endeavor to make a sort of compromise, trying to retain some of their most deeply-rooted convictions and mix them harmoniously with new views. Metallic mercury, however, will not mix with water, and there is a similar incompatibility between the explanations of the panspermatists and our present state of knowledge in regard to the question of the Origin of Life.
It remains for me now, therefore, to trace the different steps by which we have arrived at our present knowledge concerning the destructive effects of heat upon living matter. And to do this effectually I must refer my readers to good work done in the latter third of the last century by the acute and learned Abbé Spallanzani, while he was engaged in promulgating panspermatist doctrines against the views of our own countryman Needham, who, in those days, steadfastly proclaimed the truth and reality of "spontaneous generation"—though the philosophical doctrines by which he was influenced caused him to limit the acceptation of the phrase to what we now understand by the term heterogenesis.
I refer first of all to the work of Spallanzani, partly because he alone, of all those who have adopted panspermatist views and have taken part in this controversy, has fairly and fully faced the question of the degree of heat which proves fatal to various living things, by making it the subject of direct investigation. Others who have since defended similar views (including Pasteur in France, and Huxley and Sanderson in this country) have not made the thermal death-point of living matter a special subject of investigation, and have more or less distinctly confounded the issues of this question with that of the cognate though really distinct problem, as to whether certain infusions could themselves prove mother-liquids, and give independent birth to living matter. Dire confusion has thus been produced. A problem of a very simple nature has been made to appear very complex, while those who are able clearly to understand the real nature of the question at issue are left to marvel why the followers of Spallanzani have never ventured frankly to deal with the question of the limits of vital resistance to heat. Certainly they have displayed, to say the least, a strange sluggishness in reference to this exceedingly important problem. But apart from the fact that no panspermatist, or declared opponent of spontaneous generation, since the time of Spallanzani, has fully and directly experimented upon this subject, I am all the more induced to call the reader's attention to the abbé's treatment of it because, with some few exceptions, his investigations were conducted in a manner which cannot be improved upon at the present day, and because his reasonings upon the subject are characterized by great sagacity and fairness—allowance being made for the actual state of knowledge in his time. The work of the learned abbé to which I shall especially refer is entitled, in the admirable French translation by Jean Senebier, "Opuscules de Physique, Animale et Végétale," the translation itself having been published at Geneva in 1767.
Reflecting upon the import of the experiments of his own that he had just recorded, in which living organisms were found in closed vessels containing infusions of certain vegetable seeds after these closed vessels had been immersed in boiling water for half, or, in some cases, nearly three-quarters of an hour, Spallanzani frankly avows (p. 48) that if the first of the new organisms had not come into being by some such independent method as that suggested by Needham, they must have appeared either because certain "germs" from which they had been derived had been able to resist the destructive influence of boiling water for nearly three-quarters of an hour, or because, after the cooling of the closed vessels, some of the organisms observed had passed from the air through certain imaginary pores of the glass. At the first glance these seemed, as he says, "deux suppositions également impossibles, ou du moins très difficiles à concevoir." For very excellent reasons, not difficult for the reader to imagine, the abbé then points out that the latter hypothesis, at all events, is entirely untenable. The question thus became one of the simplest description. If no good reason could be found in support of the seemingly improbable supposition that the experimental results referred to were to be accounted for by a survival of germs, then, as he confessed, he must admit the fact of an independent and germless origin of living things. If, on the other hand, it should appear probable that germs or reproductive particles of living things could survive the influence of such a prolonged immersion in boiling fluids, he would not feel at all bound on the strength of his previous experiments to believe in the independent origin of living matter. This simple issue was fully realized by Spallanzani, and, acting in accordance with the most obvious of scientific principles, he carefully sought for fresh evidence by means of well-directed experiments, in order to guide him toward a conclusion as to whether germs of living things could or could not have resisted the action of boiling water for more than half an hour.
He approached the question in the following manner: "Can one," he says, "find any proof sufficient to banish, or, at all events, to diminish one's natural repugnance to admit that the germs of animalcules of the lowest order have the power of resisting the action of boiling water? In reasoning from the germs or eggs of animals with which we are acquainted, would it be difficult for us to imagine animalcules having this peculiarity? It is true that we are not acquainted with any eggs endowed with such properties. I have already considered this subject in the ninth chapter of my Dissertation. I there show how several kinds of eggs of insects—not to speak of eggs of birds—perish under a heat less than that of boiling water. I have shown also that the seeds of plants are destroyed when they are exposed to the heat of boiling water, and that even those whose outer coat is of the hardest description are not thereby spared." But he goes on to say, as he had only been able hitherto to make his observations on a limited number of eggs and seeds, there was the chance that more extended observations might reveal some capable of resisting this generally destructive influence. He says he had never lost this hope, with regard to seeds more especially, since he had seen a statement by Duhamel to the effect that some grains of wheat had germinated after having been heated in a stove to a temperature above the boiling-point of water. And as there is a considerable resemblance between seeds and eggs, Spallanzani was led to hope that something of the same alleged extraordinary capacity for resisting heat might be possessed by the eggs or germs of such organisms as make their appearance in previously boiled fluids. He was therefore stimulated to undertake fresh observations upon eggs and seeds generally, with the view, on the one hand, of ascertaining the precise temperature which proved fatal to each kind, and, on the other, of finding out whether these eggs or seeds were capable of resisting a greater degree of heat than the several animals or plants to which they belonged.
This latter part of the inquiry was rightly deemed by Spallanzani to be of great importance and capable of affording him much guidance toward the proper interpretation of his other experiments. He had already determined that the lower infusoria themselves are killed at a temperature of 34° Réaumur, or 108½° Fahr.; and now having found that such organisms would appear within closed vessels previously subjected to a temperature of 212° Fahr., owing, as he was inclined to think, to a survival of their germs, Spallanzani was anxious to ascertain whether he could gain sufficient support for this hypothesis — that is, whether the difference in the capacity of resisting heat, imagined to exist in this case between parents and germs, could be justified by the establishment of similar differences in heat-resisting capacity between other parent organisms and their germs.
In carrying out these inquiries, Spallanzani adopted the following method (p. 53): He placed the eggs, seeds, or organisms, in a vessel containing cold water, into the upper strata of which was immersed the bulb of a thermometer. The water was then heated slowly, and when the thermometer indicated that the temperature had been attained, whose effect it was desired to test, the eggs, seeds, or organisms, were at once withdrawn and placed, under suitable conditions, in a separate vessel. The effects of different grades of heat upon the objects experimented with were thus estimated, and the temperature in successive trials was mostly made to differ from that last employed by 5° R. Operating in this way, and, in the case of eggs or seeds, subsequently taking great care to place those used in the different trials under similar conditions, alike favorable for germination or development, Spallanzani obtained the following results:
Of frogs' eggs only an extremely small number developed after having been simply raised to the temperature of 131° Fahr., while not one developed which had been heated to 145° or upward. Tadpoles produced from similar eggs all perished at 111°, and the same temperature likewise proved fatal to the parent frogs from which the eggs had been derived, as well as to aquatic salamanders and to fish with which experiments were made.
Silk-worms' eggs, and the eggs of the elm-moth (Papillon de l'Orme), developed less and less frequently when successive batches were heated to temperatures approaching 1441⁄2°. When they were actually submitted to this heat, all perished, though the highest temperature followed by development is not recorded. Silk-worms themselves, as well as the caterpillars of the elm-moth, were uniformly killed as soon as the water in which they were immersed attained 1081⁄2°.
Eggs of the common blow-fly only developed in very small numbers when raised to the temperature of 135°, while all perished at 140°. The larvæ developed from these eggs all died, as those of the silkworm and elm-moth had done, as soon as the temperature of the water rose to 1081⁄2°. Other adult larvæ, of the same species with which experiment was subsequently made, perished at the same heat.
In addition, Spallanzani experimented with some aquatic organisms, though he was unable to discover, and therefore to experiment with, their eggs. Thus leeches perished at 111°, and the Nematoids known as "vinegar-eels" at 113°. Other aquatic worms were killed at 111°, while water-fleas died at 107°.
So far, therefore, Spallanzani's results were most uniform; the different kinds of eggs were killed by mere momentary exposure to a temperature of about 140° Fahr., while the animals to which they were related perished at or about 110°.
The abbé next turned his attention to the power possessed by plants and their seeds of resisting the action of heated water. These observations were conducted in the same manner, though only the roots of the plants were immersed in the water while it was being heated. The plants were afterward carefully replaced in earth. Much care was taken when the seeds were sown to keep the batches distinct from one another, and to place them as much as possible under the influence of similar conditions.
Spallanzani's first experiments were made with the seeds of the chick-pea, lentil, wheat-grass, flax, and clover. The water was heated slowly, and the seeds were taken out as soon as the desired temperature was attained, so that there was only a momentary exposure to the temperatures about to be cited. Of those which had been exposed to 190° Fahr., many did not germinate; still fewer of the seeds that had been exposed to 201° produced young plants, while, of those heated to 212, not one germinated. After the young plants (developed from seeds heated to lower temperatures) had grown for thirteen days, their capability of resisting heat was tested in the manner described, and with this result. Those whose roots had been momentarily exposed to 156° continued to live after they had been replanted, while those whose roots had been exposed to 167° and upward speedily dried up and perished, although, like the others, they had been replanted in carefully-watered earth.
These were the only complete experiments made by Spallanzani with plants and their seeds; but many other kinds of seeds only, including those of the broad-bean, barley, kidney-bean, maize, vetch, spinach, beet-root, turnip, and mallow, were also exposed to the influence of heat while packed in dry sand. Although this method is less exact and trustworthy, and is one with which we are not now concerned, still it may be stated that only four of the numerous seeds with which experiment was made after this fashion survived their brief exposure in the dry state to the temperature of 212°; all the others failed to germinate.
The abbé's researches, therefore, taught him three things: 1. That eggs can endure a decidedly higher degree of heat than that proving fatal to animals of the kind from which they have been derived; 2. That an analogous difference exists between seeds and plants in respect to their capacity of withstanding the action of heat; and, 3. That seeds and plants can resist higher grades of heat than eggs and animals respectively.
After calling attention to these conclusions, Spallanzani said (p. 64): "Of course I am far from pretending to explain these results; I know the difficulty of the undertaking, and will only venture a few conjectures, at most, letting them go for what they are worth, and leaving every one free to think as he pleases." As his conjectures, however, cannot be much improved upon at the present day, I may as well call the reader's attention to them by briefly pointing out their nature.
At the first glance, the abbé says, the superior power of resisting heat displayed by eggs and seeds, as compared with animals and plants, might be supposed to be due to the latter feeling the effects of heat more rapidly, owing to their being free from those envelopes which inclose the egg or the seed. But the weight of this supposed reason soon disappears, in the case of eggs at all events. Looking to the thinness of their investing membrane, this supposition, as Spallanzani says, "seems very improbable indeed, when we consider how easily and how rapidly fire penetrates so thin a layer of matter." He quickly dismisses, as even more improbable, the notion that the smallness of the germ or egg can act as its safeguard by rendering it less amenable to the influence of heat. Having thus cleared the ground, Spallanzani states what seems to him to be the principal reason of the difference observed. We ought to reflect, he says, upon the difference between the life of an animal in its egg stage and its subsequent life as a developed organism. For, however deficient our knowledge may be upon this subject, we may feel assured that life shows less of the characters of life in the egg than in the organism which is born from it. The life of the egg is "very feeble"—"its life has less of life." And then Spallanzani asks whether the fact of this life of the embryo within the egg being "so small and so feeble"—being "a life which deserves so little the name of life"—may not be the reason that eggs are able to bear the influence of heat better than the developed organisms whose life is more active and complex? He believes this to be the principal reason of the increased power of resisting heat displayed by eggs, and in support of it calls attention to the fact that many animals (as well as plants), when the rate of their vital phenomena is lowered, during winter sleep, are much better able to withstand many injurious external influences than when they are displaying to the full all the manifestations which constitute their "life." Animals, such as frogs and salamanders, for instance, live longer after and resist the effects of injuries better, when they have been incurred during the benumbing cold of winter rather than at periods when these organisms have been full of life and activity.
A similar difference obtains between the degree and complexity of the life of seeds as compared with that of plants, and this difference may in part similarly explain the superior power of resistance to heat shown by seeds—since here, also, among plants, we find that ability to withstand hurtful influences, generally, increases as their life becomes more sluggish. Thus Spallanzani says, "One may say that in winter plants live less fully than at other seasons, and during this period they are also much less liable to perish when they are plucked from the ground or unduly pruned, than if they had been treated in the same manner during summer."
Again, while a difference of the same kind may in part be cited as the cause of the less injurious effects of heat upon seeds and plants as compared with that which it exercises over eggs and animals respectively, Spallanzani points out that this difference between eggs and seeds is only in part due to the fact that the outer coats of most seeds are much harder than those of eggs, since the envelopes of some seeds which are only killed at a temperature near 212° are not harder than the shell of an egg which is nevertheless killed at the much lower temperature of 140° Fahr. This difference is explicable rather, according to Spallanzani, by the fact that the fluids contained within the egg are so much more abundant than those within the seed. In cases of short exposure to heat the animal embryo is thus more easily killed than the vegetable embryo, because its greater moisture causes it rapidly to experience the full effect of the heat, which the seed may possibly escape.
Now, then, for the application of the facts, toward the interpretation of Spallanzani's other experiments in which the lowest organisms appeared in closed flasks whose contents had been exposed to the temperature of boiling water for half an hour. Certainly the germs of such animalcules could not be supposed to have survived such an ordeal if they are to be compared with the eggs of animals, whose death has been brought about by momentary exposure to a temperature far short of the boiling-point. The supposition would, however, seem more possible, if, instead of comparing these germs with the eggs of animals, one regarded them as belonging to the same category as the seeds of plants. Spallanzani frankly admits that they would seem to be more allied to eggs than to seeds, though he attempts to bridge the gap by saying that certain eggs are known (to which these germs may be allied), in some respects resembling seeds. Such eggs "become dry, are preserved in this state, and then develop like seeds after they have been placed in some damp medium.... Why, then," he adds, "may not the germs of the lowest kind of animalcules be possessed of a similar nature?" He next (pp. 69-73) adduces various considerations which led him to consider this view as more and more probable, though none of them would be regarded as very relevant by physiologists of the present day. The space at my disposal will not permit of my following him into these details––the reader curious on this subject must therefore consult Spallanzani's work for himself.
The posítion of things about a century ago, therefore, was this: Not a single living thing, egg, or seed, had been shown to be able to resist, when in the moist state, an exposure to boiling water for a single moment. All naturally moist forms of living matter with which experiment had been made had been shown to be killed by a much lower heat, that is, at a temperature of about 140° Fahr., or less. And, in order to account for the appearance of the lowest animalcules in previously-boiled fluids, Spallanzani assumed—1. That these unknown germs were of the nature of seeds rather than eggs—seeing that they were capable (as he supposed) of undergoing desiccation with impunity, and that this dryness conferred upon them the greater power of resisting heat which characterized seeds. Nay, further—2. Although no seeds could be shown to be able to resist the influence of boiling water, Spallanzani assumed that these unknown seed-like germs might be able to do so. Thus alone was he able to continue in the panspermatist faith—on the strength of these assumptions only, could he refuse assent to the probability of a germless origin of living matter, more or less after the fashion suggested by Needham and others. It will, therefore, be interesting for us now to consider how far the progress of science tends to confirm or reverse Spallanzani's assumptions.
- From author's advance sheets.
- Heated in all probability in the dry state. But it is well known that seeds and desiccated animals can resist the influence of heat much better in the dried state than when they are thoroughly moistened and then heated, and it is as to the effects of heat upon living matter under the latter conditions with which we are at present concerned. For this reason, therefore, I shall not dwell upon those experiments of Spallanzani, in which he heated seeds in the midst of dry sand — these experiments lie outside the boundaries of our present inquiry.
- Spallanzani's argument thus naturally suggests the notion that many of the seeds with which he experimented required a high temperature to kill them, merely on account of their dryness. If the seeds had been well soaked in cold water beforehand, so as to have thoroughly moistened them, might they not have been killed at a much lower temperature—that is, only a little, if at all, above 140° Fahr., or the temperature which proved destructive to the more moist animal germs? Facts which will be subsequently mentioned, since ascertained by Max Schultze and Kühne, would seem to render this very probable, and compel us to regard Spallanzani's experiments with seeds as needing repetition with the modification above suggested. The plants also, like the animals, should have been wholly instead of partially immersed in the heated water.