Ponderable bodies are endowed with common or general properties, and likewise with particular or secondary properties.

The properties of atmospheric contagion, under its various titles, have been noted in the preceding chapter. They have been attributed to it, by the most eminent writers on the subject, and are such as are assented to, by most medical men of the present day.

Its origin, the sphere of its activity, and the means by which it may be destroyed or neutralized, have there been alluded to. In the extracts given, and in the current medical literature of the present time, it is spoken of, as an agent of whose existence there is the utmost assurance.

The reader who has not already thought upon the subject for himself, but has, as is almost universally done, in reference to this agent, taken the whole case, as one fully ascertained, and settled upon fixed principles, will doubtless be surprised to hear, that it is the decided opinion of a member of the medical profession, that the doctrine of atmospheric contagion presents no sufficient evidence of its truth; that he is in possession of facts connected with the occurrence of disease, which render it probable, that other and efficient causes of disease have been thrown aside, to make room for that agency, and that he is convinced, from the results of experiments on contagious poisons, and from a minute inquiry into their nature, that it (that is, atmospheric contagion) does not exist. Perhaps he should regret that he has not been able to see the question in the same light as his brethren. He has felt unwilling to espouse singular opinions; he has therefore been patient in the inquiry, and it has been only from the consideration, that a great medical truth was concerned, that the progress of the science might possibly, thereby, be promoted, and that the comfort of the patient, and the ease of mind, of the public, might be advanced, that he has been induced to lay his opinions before the world.

Regarded as a physical agent, atmospheric contagion has never been detected, and its presence has been inferred merely from the observation of what have been supposed its effects. It has certainly never been unequivocally manifested to any of the external senses. It has never been seen combined with the atmosphere, precipitated from it, or attracted therefrom, to solid bodies.

It might be supposed, however, from common parlance, that it has often made itself known to the sense of smell; but while nothing certainly proves that the impressions made on the nasal organ arise from atmospheric contagion, many circumstances induce at once the belief, that they proceed from common impurities.

The atmosphere in a sick chamber sometimes certainly has an odour, but it is certainly more logical to attribute this to the presence of impurities, whose presence there is no room to doubt, than to an agent whose existence under any circumstances has never been proven.

Had contagious matter the power of diffusing an odour through the air, it is probable, that would be constantly the same, in all cases of the same disease, and that each disease would have its own peculiar odour: but this correspondence is not found.

It is not desired to prove, that atmospheric contagion does not exist, because it cannot be detected by the senses.

Many agencies exist, which, under ordinary circumstances, are beyond the cognizance of the external senses; but in general they make themselves manifest to one, or other, under some conditions. The electricity of the air is neither seen nor felt under ordinary circumstances; but that agent is capable of being collected from the atmosphere in such quantities as are cognizable to the eye. Now, under any manner of circumstances, contagion has never been recognised by the senses—and it has never been detected by chemical experiments.

It is surely not unfair to expect, that, if a contagious poison, a palpable matter such as is contained in a small-pox pustule, is transformed to the vaporic state, or taken into the atmosphere, that the air so impregnated will be marked by some qualities, beyond those of simple, pure air. Perhaps air in which it is disseminated should have an odour, and perhaps that odour should be of a peculiar kind, in each disease. Should it not also be marked by some effects, constant and uniform, upon the human body, such as mark the career of such like agents in a palpable form, when applied either immediately, as by touch, or mediately, as by fomites? Perhaps it may not be deemed unreasonable to expect, that atmospheric contagion, did it exist, would produce its peculiar effects, as constantly, or nearly so, as a palpable contagious poison. But how different is the fact. If a hundred persons not formerly vaccinated, have the palpable contagion of cow-pox matter inserted under the skin, the probability is, that, if the matter is good, and the operation is skilfully done, 90 or 95 will be duly affected with the specific effects; whereas, when a hundred persons are exposed to the atmosphere of fever, and when these persons, too, have not before had the disease, perhaps not one, or at most not above two or three will take the distemper, unless the air has become extremely vitiated; and then the probability is, that it is so, not in consequence of the presence of specific contagious virus, but of gross impurities, and the consumption of the more vital parts, as in the case of the Black Hole of Calcutta, where putrid fever attacked all who survived their confinement, certainly not from the action of contagious poison.

Animal substances are the results of still more delicate processes, and of a more refined organization (than vegetables); and the balance of affinities, by which they exist, is disturbed by still slighter causes.

For the present, the argument drawn from the actual observation of the origin and propagation of disease, against the doctrine of atmospheric contagion, will be waived, and it is proposed here, before going farther, to inquire, whether the case may not be settled by a reference to the history of analogous agents, and to the results of experiment.

It is proposed here to inquire if it is likely, judging from their chemical constitution, that palpable contagious poisons, such as the matter of small-pox, may be disseminated through the air, without chemical changes being effected upon them, that must be destructive of their peculiar properties.

The palpable contagious poisons are products of the blood, formed therefrom, by the nicest processes. They partake of the nature common to all animal products; are, like them, prone to putrefaction,—and, like them, are of a very compound nature.

They are animal products: now it is a well known fact that almost all animal products are fixed—that is, incapable of being volatilized or disseminated in air, unchanged in chemical constitution.

Gelatin or animal jelly; albumen, or what is much the same, the white part of an egg; fibrin or muscular fibre, and the like, are never known to be in the vaporic state, or commingled with the air. They are incapable of assuming the aeriform state, not in virtue of a character peculiar to them, but on account of that nature they share in common with almost all animal principles, which precludes the possibility of their being volatilized. No experiment has ever been made which can show that the principles specified may be diffused through the air.

When exposed to the air for even a short period, decomposition takes place, and their original nature is totally subverted.

Their elements are held together by affinities too feeble to admit of their particles being separated by air, without new combinations being formed.

If heat be applied to them, immediate destruction takes place; if they be kept moist, and in merely a moderate temperature, putrefaction or fermentation, in the proper sense of the terms, occurs; if carefully dried and exposed to the atmosphere, they remain little altered, for a considerable time; but at length fundamental changes, though operating slowly, entirely change their nature.

It cannot be shewn that contagious poisons are less animalized than the products alluded to.

Is it ascertained that contagious poisons, unlike other proximate animal principles, enter into the aeriform state?

Putting aside the loose and rash statements current upon the subject, as unworthy of notice, there can be no doubt that, in the whole history of those poisons, no fact is known, that can legitimately be held as proving, that they possess such a property, or of giving the idea any degree of countenance.

On the other hand, many facts are known, which are adequate for the refutation of these statements, and that are sufficient to put the case beyond a doubt.

Small-pox propagates by a contagious poison, eliminated from the blood, and found in the pox or pustule.

It is known to every one that it affects, by contact, hence the practice of inoculation, which is nothing more than the inserting, under the skin, a little of that agent, a practice which has been in use among the negroes of Africa, since, or before, the introduction of the doctrines of Mahomet.

Many physicians, perhaps almost all, believe that it, the poison, may be diffused through the air, and in that situation produce its wonted effects; but evidence is submitted to shew, how questionable that is: and it is conclusive, as far as negative evidence can go.

The following experiment was performed by Dr O’Ryan of Lyons.^[2] The force of its results, and their tendency, cannot be overlooked.

“A dish containing lint saturated with matter taken from the natural and the inoculated small-pox, was placed upon a table, whose diameter was three feet, and children who never had the disease, and never were inoculated or vaccinated, were placed around it, and kept there for some considerable time; yet none of them were seized with the disease.”

“He also exposed children within two feet of a child affected at the time with the inoculated small-pox, for an hour daily, for fourteen days. None of the children were affected, and all were successfully inoculated two months afterwards.”

We are acquainted, too, with many cases of small-pox, where the houses in which they were, were visited by many persons, some of whom had not been vaccinated, or inoculated, and yet the disease did not spread to them; and in those instances, where the distemper did spread, only some, and not all, who were liable, were affected, as would have been the case, had the matter been inserted under the skin.

Perhaps, in reference to this contagious matter and to others, it may be said that they were not favourably situated for acting. Heat, moisture, and the passing to and fro, of air, must certainly assist the assumption of the aeriform state; and a more favourable opportunity cannot be obtained, than the contagious matter of small-pox pustules has, in the mouth of the patient, where it almost always is observed. That situation is perhaps even more favourable than that of the matter operated on by Dr O’Ryan. Yet it is known, (and we are prepared to shew cases) that persons liable to the disease have breathed in the same apartment, and have not taken the distemper. We know, too, of many cases, where persons have been attacked under such circumstances, but that has probably arisen from actual contact with the matter, or exposure to those general and widely-spread influences productive of that pestilence, that undoubtedly exist. But it is not necessary for our purpose, that all should escape, but, that any should not suffer. It is enough that those who escape, are more, in proportion, than those who resist the action of the palpable poison, when inserted into the system by inoculation.

With respect, also, to the disease produced by the insertion of cow-pox matter, or, in other words, by vaccination, as it is called; nobody ever heard of it being propagated through the air. It is feared that it would be a very inefficient mode of vaccinating, to bring the child to be vaccinated, into an atmosphere, to which was exposed an arm with a cow-pox. He who would propose such a plan would be laughed at by every old woman; and what is held as so absurd and ridiculous in respect to cow-pox, cannot be very wise in reference to small-pox, plague, scarlet fever, and the like. There are other diseases, too, which undoubtedly are propagated by palpable contagious poisons. Yet were any person affected with them, to whisper, that a contagious atmosphere had been the occasion, they would be held as using no small liberty with the credulity of the medical adviser.

There is yet another palpable contagious matter to which reference must be made,—that of itch. The only known way by which that disease can be propagated, from one to another, is by palpable or contactual contagion.

Many medical men are in the daily practice of seeing and examining such cases, yet they seldom or never are affected with it. Any caution directed against the operation of that contagion, is addressed exclusively to contact, never to the atmosphere.

The plague, according to the very best authorities, is undoubtedly marked by the elimination of a matter capable of producing the same pestilence, when applied in a palpable form, to the body of another. The plague has been produced intentionally by inoculation, and may be propagated at pleasure.

Dr Patrick Russell was satisfied, from the observation of much of that pestilence, that the atmospheric contagion did not extend the distance of four feet; and there is much room to think that, if he had extended his inquiry farther, that had he been aware how unusual it is for a proximate animal principle, as contagious matter, to take on the aeriform state, he would have arrived at the conclusion, that it did not only not exist, at the distance of four feet from the patient, but that it did not exist at all. Had he gone that length, he would not have created any more difficulties, to be explained away, than were made by laying down for it, such a limited range of operation, for there would, it seems, be little difficulty, in general, in discovering, that persons who had approached so near as four feet to the patient, had come in contact either with the sick himself, or the matter of the sores attached to clothes or other bodies.

We know of no facts capable of proving that the matter of plague is diffusible through the air; and the very evidence of Dr Russell, which was used by him to prove the limited range of atmospheric contagion, may be used to lend countenance to the position, that it does not exist at all.

The evidence was this:—Dr Russell was in the practice, at Aleppo, of examining plague sores from a window four feet from the patient, yet he suffered not from that pestilence.

Scarlet Fever is a disease universally held to be one of those propagated by a contagious principle.

It is commonly believed that a contagious poison is eliminated in the course of this disease, similar to that of small-pox.

Its history is marked by this remarkable feature, peculiar to acute contagious diseases, of attacking the same individual only once; and the disease is accompanied by a peculiar eruption, which may, without impropriety, be supposed to contain the said contagious poison. This eruption is uniform in the time of its appearance, its duration, and decay, like the other eruptions of other contagious diseases. On all these accounts, the Author is disposed to assent to its possession of the contagious poison;—and that will be taken for granted.

Connected with this view, is an observation made by Dr Sidey, of Edinburgh, in a paper contained in a late Number of the Edinburgh Medical and Surgical Journal, on Scarlatina, as lately prevalent in that town. It is to this effect, that he found that the disease, when characterized by a distinct eruption, attacked several members of a family more frequently, than when it wanted that symptom.

We will inquire whether persons exposed to an atmosphere containing one sick of that disease, take that distemper as uniformly, as those take the respective diseases of those palpable contagious poisons which may be inserted under the skin.

During a most severe and mortal visitation of that disease in Tranent and its surrounding country, which lasted from about the end of January to the 20th October 1836, many cases occurred, where brothers and sisters of children suffering under that malady, living in the same apartment, but not sleeping together, remained free of any attack whatever at the time.

Had the poison been capable of diffusion in the atmosphere, the air would have become highly contagious, and as persons were constantly inhaling it, and among them some liable to the disease, it would certainly have manifested its peculiar pestiferous influence upon them.

But the result was different, and the person exposed at that time remained quite free of it; and in the course of time, varying from weeks to several months after, went through the disease in the ordinary manner. These cases have been carefully noted and preserved.

But the Author was anxious to ascertain, by other means, whether that disease was capable of propagation by atmospheric contagion; and opportunities were not wanting.

It occurred that the matter of ulcers, in the throat, might possibly contain the contagious poison, and might be made the subject of experiment.

The following is a case in which the experiment was made.

The patient, a boy eight years old, had been exposed about three months before, constantly, to an atmosphere in which a younger brother, ill of scarlet fever, was breathing.

He had the precursory fever, and the tonsils and uvula (the parts at the back of the mouth) were almost covered with ash-coloured spots and suppurating ulcers.

A piece of linen, fixed to the extremity of a probe, was rubbed freely over the ulcers. The linen impregnated with matter and the secretions, was, within an hour or two of its being taken, exposed to the free action of the air of a small apartment, where it remained for ten days, without producing any effect, upon several persons, a good deal in the room; and among them, two children, one aged two, and the other fourteen years, who had not had scarlet fever. They respired the air occasionally and for a considerable time, on the several days.

The temperature was various. During the day being about 60° Fahr., and 40° during the night. The linen readily became dry, but was repeatedly moistened with water.

This experiment goes to shew, that the matter of the ulcers of scarlet fever is incapable of propagating the disease, through the medium of the air.

But scarcely any better nidus could be formed, for the dissemination of the matter, of the ulcers, through the atmosphere, than the sores themselves, the very place where it is eliminated; and cases have been referred to, where persons have respired an atmosphere thus liable to be acted on, with the most complete impunity.

It is not ascertained that the contagious poison is eliminated at the sores in the throat, but such seems probable, seeing that the sores are as essential and constant as the eruption itself.

Experiments might have been multiplied, but that has appeared unnecessary, as it is hoped that enough has been done to shew that the contagious poisons which have undergone our examination, are incapable of assuming the aeriform state, and, as it must seem probable, that in a point so important, they will all coincide, even those which have not been treated of here.

Their chemical constitution, as before remarked, prevents their assuming that state. Dr Henry of Manchester remarks, when pointing out the distinctive characters of animal and vegetable bodies, that “Animal substances are the results of still more delicate processes, and of a more refined organization, and the balance of affinities by which they exist is disturbed by still slighter causes;” and again says, “Instead of passing through the vinous and the acetous fermentation, they are peculiarly prone to undergo putrefaction.”

Thus, then, this great law, ascertained and settled beyond a doubt, and the results of our observations on the causes of diseases styled contagious, and of experiments on the palpable contagious poisons themselves, are opposed to the admission of this doctrine, and when we recall to memory the slender evidence, nay, the absence of any evidence at all, the conclusion almost necessarily is, that atmospheric contagion does not, and cannot exist.

With what justice may we now join with De Lolme, when he says—“There is a very essential consideration to be made in every science, though speculators are very apt to lose sight of it, which is, that in order that things may have existence, that they must be possible.”

Lest the evidence we have laid before the reader should not be so satisfactory and conclusive as it has been deemed by us, the details and results of some investigation into yeast will now be given.

It occurred to us, that it would be useful, in our inquiry respecting contagious poisons, to ascertain whether or not yeast was capable of producing its wonted effects through the medium of the air, if, in short, it was capable of taking on the vaporic state. We were led to this inquiry from the consideration, that it and contagious poisons presented points of resemblance of the most important nature, and that the history of the one might elucidate that of the others.

Yeast is the only other inanimate substance, besides the contagious poisons, with which we are acquainted, which has the property of producing a substance in every respect like itself, in short, of reproduction.

Like the contagious poisons, too, it is the result of a great and active process, which, like them, it can again produce in other materials.

Fermentation may be likened to contagious disease, and, indeed, it is not the first time contagious disease has been likened to fermentation. These diseases produce contagious poisons,—fermentation produces yeast, and again, these agents produce their respective processes.

Bodies in general, which have undergone the action at least of the active contagious poisons, are not liable to be again affected by them; so vegetable bodies, which have undergone fermentation, by means of yeast, are not liable to be again acted upon by a second application.

It is important to know if yeast is capable of assuming the aeriform state.

It is a complex substance, being compound in its chemical constitution. Did we find that it was, then it might seem probable that contagious poisons (putting out of consideration the evidence already given), might possibly be so disseminated also. It runs readily into putrefaction, and in a short time loses its power of producing its peculiar effects, that is, fermentation.

Knowing this, we were inclined to believe that it could not get into the atmosphere otherwise than in a decomposed state, and, therefore, could not act through that medium.

The question was put to a most intelligent brewer, conversant with its common qualities, and the unhesitating answer was immediately given, that it could act through the air.

Here we could not help marking the striking similarity in the bearing of the brewer, with the confidence with which medical men speak of the like property of contagious poisons—the marked taking for granted what was opposed in both instances, to the obvious evidence of chemistry, and what might be so readily tested by experiment.

He was of opinion, that if fermentation were going on in a tub in an apartment where there was a quantity of wort (liquid ready for fermentation), to which no yeast had been added, that that process would be excited from the yeast in the fermenting tub, producing its influence through the medium of the atmosphere, in short, by being dissolved in it.

As that opinion did not tally with our opinions on chemical affinity, recourse was had to experiment.

1st Experiment.

A quantity of wort, to which no yeast had been added, was put into a wide mouthed vessel, and suspended in the mouth of a large tub, containing ale in an active state of fermentation. The vessel was allowed to remain three days, and at the end of that time no more appearance of fermentation was detected, than a very slight display of frothy bubbles in the middle, nothing more than we were assured by the brewer, was wont to appear from spontaneous fermentation.

A blind devotion to his opinion might have induced the brewer to attribute to the yeast acting through the medium of the air, what was quite spontaneous, and if he had done so, how like his case would have been to that of some medical men, who unwittingly attribute to atmospheric contagion, what is spontaneous or dependent on other agencies.

From this experiment it appears that yeast is incapable of solution in the air, and of producing through that medium its peculiar effects.

But to make the result still more certain, another experiment was performed.

2d Experiment.

A wide mouthed vessel, containing a quantity of water, was suspended over some liquor, in a state of active fermentation, for the purpose of absorbing any gas or yeast, in a state of vapour proceeding from it. It was kept there two days, and then examined. Its taste was somewhat altered, and it had acquired a slight odour much resembling that of yeast, probably from the absorption of gas. It was thought, that if this water had become impregnated with yeast, that that circumstance would be rendered manifest, by producing fermentation, when added to a quantity of wort; and to determine the question, the following trial was made.

Two jugs half filled with wort, free from yeast, were placed in an apartment whose atmosphere was favourable to fermentation. To one was added the water which had been suspended over the fermenting tub, and to the other an equal quantity of pure water. They were then put aside, and secured from interference. At the end of three days they were examined. The wort to which had been added the water taken from over the fermenting tub, presented on its surface a few frothy bubbles, but not the slightest appearance of yeast.

The wort, to which pure water had been added, presented an appearance identically the same, having a few frothy bubbles on its surface, but not any other, the most trifling sign of fermentation.

Similar experiments were made at a distillery, where the facilities for their success were said to be even greater than at the brewery, and they were marked with precisely similar results.

Thus, then, it appears, as the result of experiment, that yeast is incapable of assuming the vaporic or aeriform state.

This inquiry will perhaps appear to many remote and unconnected with the proper subject of these pages, and, hence, that it is altogether superfluous; but we think differently, and are of opinion, that an accurate knowledge of that agent is calculated to be of the utmost use in forwarding the formation of a just estimate of the habitudes of the contagious poisons, which it resembles in several very important points.

It is, as before stated, the only other substance belonging to the inanimate world, whose immediate and most prominent property is that of propagating a substance identically the same—of producing, through a peculiar and uniform process, an agent possessed again of all its properties.

Some other agents may be said, under some circumstances, to propagate themselves, but it is in a very remote way, and by no means by that direct and uniform operation which marks the propagation of contagious poisons and yeast, which is obviously as well defined as germination among animal and vegetable bodies.

Heat, under some circumstances, does cause the production or evolution of heat, but that is rather an accidental circumstance, brought about remotely by the chemical operation produced, and would have taken place whatever had been the cause of that process, and is not the result of an immediate and particular property.

Vitiated air also is calculated much in the same way to reproduce itself; but, instead of being in virtue of a quality possessed by the palpable contagious poisons, vitiated air of itself produces disease, and a common result of disease is vitiated or impure air.

The close analogy subsisting between yeast and the palpable contagious poisons, it is hoped, has been fully made out; and though it is not permitted, by the rules of logic, positively to determine, that the laws which regulate the action of the one, necessarily hold with the other agents; yet, where there is no evidence of a contrary nature, the closeness of the connection lends countenance to the idea.

That analogy seems remarkably strong when it is considered, that both yeast and the palpable contagious poisons produce their peculiar effects only once upon the same object.

Many instances are known where the palpable contagious poisons have produced their peculiar effects more than once, but these deserve rather to be held as exceptions to the general law than as a proof against its existence.