Popular Science Monthly/Volume 6/December 1874/Thermal Death-Point of Living Matter II
By H. CHARLTON BASTIAN, M. D., F. R. S.,
PROFESSOR OF PATHOLOGICAL ANATOMY IN UNIVERSITY COLLEGE, LONDON.
ALTHOUGH it is doubtless true that the superior dryness of seeds does enable them to resist the influence of heat longer than moist eggs are able to do, and therefore also enables them apparently to resist for a brief period a temperature notably higher than would have proved fatal to them had they been in a moist state—it is altogether another question when we have to decide whether moist Bacteria or their germs are endowed with this seed-like property of developing after desiccation. To maintain his position, Spallanzani was compelled to assume that they did possess this potentiality. Modern science, however, on the basis of experiment, declares that they have no such property. We are told most unreservedly by Prof. Burdon-Sanderson, not only that "the germinal particles of microzyms (Bacteria) are rendered inactive by thorough drying without the application of heat," that is, by mere exposure to air for two or three days at a temperature of 104° Fahr., but also that, "fully-formed Bacteria are deprived of their power of further development by thorough desiccation." Thus is the most important assumption made by Spallanzani swept away, and with it all the strength that his position may have appeared to possess. Neither he, nor any of his followers, can hope to save their germs from the full action of heat by assuming the preëxistence of a protective desiccation, when they are told, on the unquestionable authority of Prof. Sanderson, that such desiccation would be in itself destructive to them.
We are left, therefore, now face to face with only one other question. Has the progress of science, it may be asked, since the time of Spallanzani, in any way tended to strengthen the possibility that Bacteria-germs or any forms of living matter in the moist state can resist the destructive action of boiling water, even for two or three minutes? And to this question a negative answer may be unreservedly given, since the progress of science has shown, on the contrary, that such a supposition becomes more and more improbable in the light of all uncomplicated investigations bearing on the subject. To these results of modern research I must now call the reader's attention.
In the first place, the specific question with which we are more immediately concerned, as to the thermal death-point of Bacteria and their germs, has itself been answered by most decisive experiments. As the writer has elsewhere already shown, all direct experimentation on this subject leads to the conclusion that Bacteria and their germs, whether visible or invisible, are killed by a brief exposure to a heat of 140° Fahr. in the moist state. Thus Dr. Sanderson's experiments having proved that the germs of these organisms are, as regards their ability to withstand desiccation, related to eggs rather than to seeds, the writer's own experiments tend to strengthen this resemblance by showing that these Bacteria-germs also (like the eggs with which Spallanzani experimented) are invariably killed at a temperature of about 140° Fahr.
Although, therefore, my experiments are not favorable to Spallanzani's assumptions, they are entirely in accordance with his experiments. The thermal death-point ascertained by him for the eggs of Insects and of Batrachia, agrees almost exactly with that which I have established for Bacteria-germs, although at the time my own experiments were made I was unaware of these particular results obtained by Spallanzani.
Is there, then, any thing in this fact concerning Bacteria and their germs at all at variance with what we might have been led to expect from our knowledge about the capacity for resisting heat shown by other kinds of living matter? Here, again, a negative answer may be unreservedly given. The grounds for this opinion must, however, be set forth, and, in dealing with this important question, I will range what I have to say under the following heads: 1. The results obtained by many other investigators, working quite independently of one another, (and in many cases also without distinct reference to the Origin-of-Life question), all go to show that different kinds of living matter are killed when in the moist state at or below the temperature of 140° Fahr. 2. The only known exceptions to this rule are cases of a special kind differing altogether from those with which we are at present concerned. 3. Our knowledge concerning the thermal death-point of Living Organisms and of Living Matter is remarkably harmonious, and is in accordance therefore with what we know concerning the unity of living matter generally. 4. The assumptions entertained by some, in support of their notion that living matter unaccustomed to the influence of heat is able to resist the destructive action of boiling water, are of the most frivolous nature—alike unsupported by experiment and contradicted by all ordinary experience.
1. Liebig proved that sugar-yeast (Torula cerevisiæ) entirely lost its power of growth and germination at 140 Fahr. It has been ascertained by Tarnowski, after numerous experiments conducted, as Sachs says, "with all possible precautions," that spores of Penicillium and other common fungi, also most closely related by nature to Bacteria, "entirely lost their power of germinating when heated in their own nourishing fluids" to a temperature of 131° Fahr. Again, it has been ascertained by Dr. Timothy Lewis that the germs of tape-worms are invariably killed at the temperature of 131°, while Prof. Mantegazza has shown that the male reproductive particles of frogs are killed by exposure to the same heat. So far, therefore, concerning germs, in addition to what I have already mentioned about Spallanzani's observations upon the eggs of Insects and Batrachia. Turning now to adult organisms of different kinds or to their elemental parts, the following facts maybe cited: Pouchet found that all kinds of Ciliated Infusoria were certainly killed at 131° Fahr., and while confirming this observation the writer found that a brief exposure to this temperature always sufficed to kill Amœbæ, Monads, Euglenæ, Desmids, Rotifers, Nematoids, and other minute aquatic organisms. The writer did not try to ascertain what was the lowest temperature which would prove fatal to these organisms, though this has been done by other observers. Spallanzani, for instance, ascertained that Ciliated Infusoria, Waterfleas, Leeches, Nematoids, and other worm-like creatures, all perished at 107°-113° Fahr.; while Max Schultze, and Kühne, in part working over the same ground, have quite recently fixed the limits for such organisms at temperatures varying between 104° and 113° Fahr. At these temperatures the protoplasm entering into the formation of such organisms as well as that of the tissue-elements of higher animals was not only killed, it became coagulated and assumed the condition named by Kühne "heat-rigidity." Both Max Schultze and Kühne also found that the protoplasm of plant-cells with which they experimented was always similarly killed and altered by a very brief exposure to a temperature of 1181⁄2° Fahr. as a maximum. All accurate new observations, therefore, go to prove that different kinds of living matter, whether in the form of germ or of developed organism, are killed by a brief exposure in the moist state to a temperature at or below 140° Fahr.
2. So far I have been referring to the influence of heat upon living matter when it is suddenly applied to an altogether unaccustomed extent. This is the mode of operation with which we are especially concerned, as, with the view to the interpretation of experiments on the Origin-of-Life question, we wish to know the effects of great heat upon organisms accustomed to ordinary atmospheric and aquatic temperatures. On the other hand, it should be pointed out that organisms have been found living in hot springs at temperatures very considerably above those I have just been quoting; although the very highest of the temperatures, under the influence of which living things have been reported as existing in thermal springs, is still a few degrees below the boiling-point of water. The various observations that have been made upon this subject have been collected and criticised with much care by Prof. Jeffries Wyman, to whose paper I would refer the reader. The most remarkable instances of this kind, in which Confervæ, or allied organisms, have been met with—that is, the highest temperatures cited which are at all trustworthy—are thus summarized by Prof. Wyman: "The statements we have quoted," he says, "give satisfactory proof that different kinds of plants may live in water of various temperatures, as high as 168° Fahr., as observed by Dr. Hooker in Sorujkund; 174° as observed by Captain Strachey in Thibet; 185° as observed by Humboldt in La Trinchéra; 199° as observed by Dr. Brewer in California; and 208° as observed by Descloizeaux in Iceland." As we have no grounds for criticising these observations, we are bound to look upon them, provisionally at least, as correct and taken with all due care, though it is only fair to add that both Max Schultze and Cohn appear to be not altogether satisfied with some statements of the same kind. Such instances, if thoroughly accurate, may perhaps be taken as examples of the highest temperature which it is possible for living matter to endure, even where it has been inured to its influence in the most gradual manner. And the real point of view from which these facts should be regarded is, indeed, pointed out by Prof. Wyman when he says: "Having become adapted through a long series of years to their surroundings, such organisms may be supposed to live under circumstances the most favorable possible for sustaining life at a high temperature. It is a well-known physiological fact that living beings may be slowly transferred to new and widely different conditions without injury; but if the same change is suddenly made, they perish. In the experiments made in our laboratories, the change of conditions is relatively violent, and therefore liable to destroy life by its suddenness."
3. If we omit, therefore, the facts concerning the existence of living organisms in thermal springs which are altogether peculiar, and which lie outside the boundaries of our present inquiry, all that we know about the unaccustomed influence of high temperatures upon living things can easily be shown to be even more harmonious than it may at first glance appear. We have only to bear in mind two or three general principles in order to be able to harmonize the several experimental results arrived at with the now very generally admitted doctrine as to the oneness or generic resemblance existing between all forms of living matter. We must bear in mind, first of all, the consideration enforced by Spallanzani, that there are different grades of vitality, or, in other words, forms of living matter which exhibit more or less of the phenomena known as vital, and that of these forms those which exhibit the most active life are those which would be most easily killed by heat. Thus we should expect the latent "life" of the germ, egg, or seed, to be less easily extinguished than the more subtile, and, at the same time, more active life of the fully-developed tissue-element or organism; and we should also expect that the vegetal element or organism would, as a rule, be less readily killed than the more highly-vitalized animal element or organism. These principles, based upon the consideration of relative complexity of life, are, however, subject to the influence of a disturbing cause, since we must also take into account, in the case of animals, whether we have to do with the elements of a warm-blooded or a cold-blooded organism, owing to the fact that custom or habitual conditions tend to render the more active tissue-elements of warm-blooded animals better able to withstand the influence of heat than similar elements of less highly vitalized cold-blooded animals. Keeping these considerations in view, therefore, we may see by the following figures how harmonious are the facts already ascertained:
TEMPERATURES AT WHICH DEATH OCCURS.
4. I now turn to say a very few words concerning the general attitude and specific statements made by those who, wishing not to give in their adherence to the fact of the occurrence of "spontaneous generation," affect to believe that Bacteria-germs or other kinds of living: matter can resist the influence of boiling water.
In the first place, it should be said that not one of these persons has striven to justify his position by scientific evidence bearing directly upon the death-point of Bacteria and their germs, while several of them have glaringly attempted to make good their position in the most unscientific manner, that is, by adducing experiments admitting of two interpretations as though they were instances only admitting of one, and then of these two possible interpretations selecting that which the experiments were not originally destined to illustrate, and which is, moreover, contradicted by other less equivocal evidence, as to the very existence of which those who adopt this course take care to say nothing. This is a kind of treason to Science of which I hope the future may prove less prolific than the past has been.
And, if we turn now to the specific statements made by those who profess to believe that Bacteria and their germs are able to resist the influence of boiling water, we discover in the first place that all who advance such suppositions find it convenient to pass unnoticed the several series of experiments by which it has been proved that Bacteria and their germs are uniformly killed by an exposure to 140° Fahr. for five minutes. My opponents find it most convenient to take no notice of these experiments, though no one has as yet attempted to dispute their cogency. They prefer to talk vaguely, as though these experiments had never been made, and to adduce various theoretical reasons whose validity they do not attempt to test experimentally. To do this, indeed, would be a vain attempt, because they must be aware that such suppositions as they advance are opposed to generally-accredited scientific doctrines, even if they have not already been specifically refuted.
The suppositions principally dwelt upon may be ranged under three categories:
(a.) It is assumed by some that the mere minuteness of the germs of Bacteria may serve to protect them from that destructive influence which heat exercises upon living matter generally. This is an old objection entirely unsupported by facts, and those who dwell upon it may be reminded that it was unhesitatingly rejected by the former chief of their school, Spallanzani, who said, "Un raisonnement de cette sorte est absolument contraire à toutes les notions que nous avons du feu." They may be further reminded that the writer's own experiments absolutely meet this objection, since they refer to the death-point of invisible germs of Bacteria just as much as to the death-point of those which are visible.
(b.) Others, without definitely committing themselves to the belief that Bacteria-germs can resist the destructive influence of boiling water when they are immersed in it, affect to believe that some germs may have escaped its influence by being "spurted" out of the fluid on to the sides of the glass when the process of boiling commenced. How any such germs could escape the moistening and destructive influence of the hot steam with which they would still be in contact these reasoners do not say, though some of them are cautious about openly suggesting an antecedent and protective state of extreme desiccation in the face of Dr. Sanderson's experiments proving that this would be in itself destructive. The futility of this reasoning has, however, been completely demonstrated by the fact that organisms will occur just as freely under conditions where no such objection can be alleged, that is, when the vessel and its contents are heated by submergence in boiling water, after it has been hermetically sealed—a mode of heating that has been occasionally adopted by different experimenters since the time of Spallanzani.
(c.) The third objection raised is no less remarkable, owing to its being similarly brought forward as an unsupported supposition in the face of much other evidence testifying to its nullity. When the writer's earlier experiments were first recorded, the public was authoritatively told by Prof. Huxley that the results were unworthy of credence, because the fact that tons of meats and vegetables were annually preserved from putrefaction by a very similar process was in itself the strongest evidence that he had in some manner deceived himself. It was never suggested or thought of, therefore, at this time, that such moist meats and vegetables were incapable of being heated through, even when pounds of them were aggregated together. It was, in fact, implicitly said that they could be so heated, and the fact of the preservation of the meats and vegetables was itself deemed to be the best evidence that all germs contained in their interior had been killed. Now that the writer has demonstrated to unbelievers, and when others have ascertained for themselves, that organisms are to be met with and that putrefaction will occur within almost airless and hermetically-sealed flasks whose contents have been previously boiled, the tactics of these unbelievers are entirely changed. Forgetting altogether their previous objection upon which they relied so long as they doubted the writer's facts, they now advance the interpretation of his results, which must carry with it its own stultification to the minds of those who have not entirely forgotten their previous position. The writer's methods are declared to be faulty for not freeing his infusions from all particles, however minute and however soft. The oracles now shake their heads, and talk with apparent learning about "the protective influence of lumps." While heat was previously supposed to be capable of operating as a germ-killer through pots of meats and vegetables, and while it has been proved to act in the same way through the thick and dry envelopes of seeds, now a pea or a minute particle of cheese, even though smaller than a pin's head, is thought to exercise a "protective influence" over imaginary germs! Such puerilities may safely be left to die a natural death, though it may be as well to remind those who trust to them, that, although they do not put their notions to the test of direct experiment, others have, for certain practical reasons, had occasion to do so. Dr. Timothy Lewis, who has been for some time in Calcutta, carrying on, in concert with Dr. D. Cunningham, important sanitary investigations, has, among other things, directed his attention to the vitality of tapeworm germs in cooked meat. He proved, first, that tape-worm germs are undoubtedly killed by exposure for five minutes to a temperature of 135°-140° Fahr.; and then, with a view of ascertaining also how far they would be likely to experience such a temperature in the ordinary process of meat-cooking, he made other important observations having considerable interest for us. Dr. Lewis found that when legs of mutton had been put into the boiler almost as soon as the water, their central temperature averaged 140° Fahr. by the time the water around them had reached the boiling-point, and that, after the water had boiled for five minutes, the internal temperature of the legs of mutton which had remained in the boiler had on an average reached 170°. This is a practical method of dealing with the question which those skeptical dreamers who talk of the "protective influence of lumps" would do well to imitate.
After this I may perhaps be deemed fully justified in quoting two very typical experiments for the consideration of those who stave oft* their belief in the occurrence of "spontaneous generation" either by relying upon insufficient reasons for doubting the influence of boiling water, or because of their following Pasteur, Cohn, and others, in supposing that certain peculiar Bacteria-germs are not killed except by a brief exposure to a heat of 227° or 230° Fahr. For, even if we could grant them these limits, of what avail would the concession be toward staving off the dreaded admission of the occurrence of "spontaneous generation," in the face of such experiments as those which follow?
Experiment I.—A strong infusion of turnip was rendered faintly alkaline by liquor potassse, and to this a few separate muscular fibres of a codfish were added. Some of this mixture was introduced into a flask of nearly two ounces' capacity. Its neck was drawn out and afterward hermetically sealed by the blow-pipe flame, while the fluid within was boiling. When thus closed the flask was about half full of fluid. It was then introduced into a digester which was gradually heated, and afterward kept at a temperature of 270°-275° Fahr., for twenty minutes, though it seems also well to point out that, if we include the time taken for the water of the digester (in which the closed flask was immersed) to attain this heat, and also again to cool down to 230 Fahr., this flask was exposed to temperatures above 230° Fahr. for one hour, as I myself carefully noted at the time. When withdrawn from the digester the closed flask was kept at a temperature of 70-80° Fahr. for eight weeks, and during part of this time it was exposed to the influence of direct sunlight. After it had been ascertained that the flask was free from all crack or fault, its neck was broken, in order that its contents might be examined. The reaction of the fluid was found to have become decidedly acid, and it had a sour though not fetid odor, as though a fermentative process had been taking place in the solution. The fluid was very slightly turbid, and there was a well-marked sediment consisting of reddish-brown fragments, and of a light flocculent deposit. On microscopical examination the fragments were found to be portions of altered muscular fibre, while the flocculent deposit was composed for the most part of granular aggregations of Bacteria. In the portions of fluid and of deposit which were examined, there were thousands of Bacteria of most diverse shapes and sizes, either separate or aggregated into flakes. There were also a large number of monilated chains, of various lengths, of a kind very frequently met with in abscesses and other situations, where pyæmia or low typhoid states of the system exist, in the human subject. There were, in addition, a large number of Torula corpuscles, as well as of brownish, nucleated, spore-like bodies, gradually increasing in size from mere specks, about 1⁄30000th up to 1⁄2599th of an inch in diameter. Lastly, there was a small quantity of a mycelial Fungus filament, bearing short lateral branches, most of which were capped by a single spore-like body.
Experiment II.—A strong infusion of common cress (Lepidium sativum), to which a few of the leaves and stalks of the plant were added, was inclosed in an hermetically-sealed flask in the same way, heated in the digester at the same time (and therefore to the same temperature), and was subsequently exposed to the influence of the same conditions as I have already mentioned in connection with the last experiment. This flask was, however, opened one week later—that is, at the close of the ninth week after it had been heated in the digester to 270-275° Fahr. Before breaking the neck of the flask, the inbending of the glass under the blow-pipe flame showed that it was still hermetically sealed. The reaction of the fluid was found to be distinctly acid, though there was no notable odor. The fluid itself was tolerably clear and free from scum, but there was a dirty-looking flocculent sediment at the bottom of the flask, among the débris of the cress. On microscopical examination (with a 1⁄12th "immersion" objective) much altered chlorophyll existed, either dispersed or aggregated among the other granular matter of the sediment, and among some of this three minute and delicate Protamœbæ were seen, varying in form, and creeping with moderately rapid, slug-like movements. They contained no nucleus, and presented only a few granules in their interior. In the same drop of fluid, and also in others subsequently examined, more than a dozen very active Monads (1⁄4000th of an inch in diameter) were seen, each provided with a long, rapidly-moving lash by which neighboring granules were freely knocked about. There were many smaller motionless and tailless spherules of different sizes, whose body substance presented a similar appearance to that of the Monads—and of which they were, in all probability, earlier developmental forms. There were also several unjointed "Bacteria", presenting most rapid progressive movements accompanied by quick axial rotations. Many Torula corpuscles and other Fungus "spores" also existed, as well as portions of a mycelial filament containing equal segments of colorless protoplasm within its thin investing membrane.
A drop of the fluid containing several of these active Monads was placed for about five minutes on a glass slip in a warm-water oven maintained at a temperature 140° Fahr. All the movements of the Monads ceased from this time, and they never afterward showed any signs of life.
These experiments are two of the most remarkable selected from several others in which even higher temperatures were originally had recourse to in order to free the fluids and flasks generally from any thing like a trace of living matter. Nothing, that has yet been alleged by way of objection to the admission of "spontaneous generation" as an every-day fact, at all affects such experiments as these. The shortest way out of the difficulty would therefore be to doubt the facts. I can assure the reader, however, that they are as true and just as reliable as those other results obtained when working with lower temperatures, which, though strongly disbelieved in at first, are now generally recognized as trustworthy. And, although these now accredited results abundantly suffice, in face of our present knowledge concerning the limits of vital resistance to heat, to establish the strongest probability of the occurrence of "spontaneous generation," yet such experiments as those which I have now recorded even still further confirm this view, since it becomes incredible that, while all known forms of living matter with, which accurate experiment has been made inevitably perish at or about 140° Fahr., the particular examples of the same forms which appear within our sealed flasks have been able to survive a much longer exposure to 270°-275° Fahr. If this were true, then indeed would the cultivation of Science be a vain pursuit—"uniformity," in fact, must be postulated and granted, or Science with humbled and sorrowful crest must retire from the field.
A word or two must be said in conclusion with reference to the interpretation which should be attached to such experiments as those just recorded. And this subject cannot be better introduced than by means of the following extract from the already-quoted and valuable paper by Prof. Jeffries Wyman. He says: "There can therefore be no certainty of the existence of spontaneous generation in a given solution, until it can be shown that this has been freed of all living-organisms which it contained at the beginning of the experiment, and kept free of all such from without during the progress of it. On the other hand, this kind of generation becomes probable, whenever it is made certain that Infusoria are generated in solutions in which the conditions just mentioned have been complied with. We say able, because their appearance under such circumstances would not amount to a proof. The absolute proof of spontaneous generation must come from the formation of living organisms out of inorganic matter. If Infusoria are generated in solutions of organic matter, independently of spores or germs, the question may be fairly raised whether we do not begin the experiment with materials in which life already exists, even though this material is not in the form of distinct organisms." Now, these last few lines, as they at present stand, tend to convey to the reader very erroneous impressions, and yet I am aware that views of the same kind are very commonly expressed, and seem to exist in an inchoate or half-realized form in the minds of many distinguished persons. It is for this reason, and on account of the authority attaching to Prof. Wyman's statements, that I am induced to take notice of this particular passage in order to attempt its rectification.
In the first place, then, under the old term, "spontaneous generation," are included two processes quite distinct from one another—namely, Heterogenesis and Archebiosis. With regard to Heterogenesis, this is merely the opposite of Homogenesis: and the latter is the name for that mode of generation or reproduction among living things which is looked upon with most respect and which is most generally known. It is the process by which "like produces like," that is, where the offspring grow into beings like their parents. In Heterogenesis, on the other hand, we have the birth of dissimilar products, the beginning of a new branch from a "life-tree," in which the offspring have no tendency to assume the parental type. This occurs, for instance, where the protoplasmic matter of an animal or of a vegetal cell becomes modified and resolved into Bacteria. Here we have to do with the mere transformation of living matter. It can, therefore, only take place where living matter preexists. And seeing that many, among whom I may especially mention Needham, Pouchet, and Trécul, have, both now and formerly, understood by the phrase spontaneous generation merely such a process of metamorphoses of living matter as is implied by the term Heterogenesis, it is very misleading to say that "the absolute proof of spontaneous generation must come from the formation of living organisms out of inorganic matter."
It seems obvious, however, that, when Prof. Wyman wrote this passage, he, forgetting the more common acceptation of the phrase "spontaneous generation," must have used it in the same sense as I now employ the term Archebiosis—in the sense, that is, of life-origination. But, even taking it in this sense, how far, we may ask, is Prof. Wyman justified in saying that its proof "must come from the formation of living organisms out of inorganic matter?"
The statement is, in my opinion, one which cannot be logically entertained by a believer in the ordinary physical doctrines of life, and consequently should be professed by no consistent believer in Evolution. Those who do not assent to these doctrines would probably never be able to believe in Archebiosis at all—to the "vitalist" life is an immaterial principle specially created, and therefore our flask experiments terminating in the birth of new organisms, if they carried with them any convictions at all, would simply be regarded by him as proving the occurrence of Heterogenesis. This is the view to which a vitalist would be driven, if he had become convinced that no germs of Bacteria, or of such organisms as are found in our flasks, could have survived the preliminary process of heating. Such a vague sort of position is not open, however, to those who believe in the now generally-accepted physical doctrines of life. They are bound to recognize the undoubted distinction which exists between mere dead organic matter and that organic matter which displays the phenomena of life. They should no more think of calling a body "living" which could not be made to display the characteristics of life, than they would call a body "magnetic" when it would show none of the properties pertaining to magnetism. If they had learned, therefore, that living matter when exposed to heat of a certain intensity became lifeless matter, the process by which new living protoplasm comes into existence among this dead organic material would be, for them, as much an instance of its new independent origin as if the process had occurred in the midst of mere inorganic elements. The term Archebiosis is therefore applicable to the process that must take place in our ordinary flask experiments where we have to do with dead organic matter, just as it is also applicable to those more primordial combinations which first gave birth to living protoplasm. The continued occurrence of an independent elemental "origin" of living matter we are called upon to believe in at the present day, though the actual steps of the process by which it takes place are unfortunately as completely unknown to us as are the steps by which its "growth" occurs whether from organic or from inorganic materials.—Contemporary Review.