Now, these salts[48] when placed in contact with animal and vegetable substances, perform the same function as in the lungs: they take a part in the combustion going on, and, as in the living body, are converted into carbonates. Left to themselves for a time, from their aqueous solution, the acids composing them finally completely disappear.
Mineral acids and nonvolatile vegetable acids, as well as mineral salts with an alkaline base, have the property, when sufficiently concentrated, to arrest the whole process of this slow combustion;[49] common salt, as is well-known, arrests putrefaction: so does alcohol.
The chemical action of certain other mineral salts is different, such as the salts of the peroxide of iron, of lead, bismuth, copper, and mercury. These are inorganic poisons. They combine with the tissues of the organs, and so destroy life. The mode of action of the poisons of prussic acid, strychnine, morphine, &c., is as yet unknown.
“But there exists a class of substances no less fatal than the preceding, originating in certain decompositions. In a preceding Chapter [(III].) we have inquired into the origin of these poisons, and shown them to originate in fermentation and putrefaction. Let us apply the chemical principles regulating these processes to organic matters, to the products of the animal economy; all the elements of these matters are derived from the blood, the most complex of all existing substances. In decomposing, a poison is occasionally produced speedily mortal when it comes in contact with the blood of the living animal. The venomous principle produced by decomposing animal bodies is not always the same: that originating in certain German sausages is quite peculiar; the person who partakes of this fatal dish dies mummified; he does not rot or fall to pieces like those who perish from wounds received in dissecting-rooms; on the contrary, he dries up and withers, but does not putrify.[50] Liebig, the discoverer of this poison and its effects on man, states that the venom is destroyed by boiling-water and alcohol, but that these do not absorb it.
Similar in the mode of action on the economy are the poisons of small-pox, plague, &c. The substances which arrest fermentation and putrefaction, also neutralize the power of these poisons; but the essence of these poisons has not yet been obtained in an isolated form, and thus nothing positive is known of its real nature. One thing seems certain; contagions, poisons and miasms are not living beings nor animalcules, any more than yeast. They may be, and probably are, produced by fermentation, but this is neither caused by nor terminates in the formation of living animalcules, to which all or any of these phenomena might be attributed.
A nice distinction has been drawn by a distinguished chemist between a contagion properly so-called and a miasm. When the disease-producing matter is the product of a disease, it is a contagion; if it be the product of putrefaction or of eremacausis of any substance, animal or vegetable,—does it act by virtue of its chemical character, and not of its condition (etat), in forming a combination, or in causing a decomposition, it is then a miasm.
The history of diseases so originating scarcely supports this view. Typhus, which at times seems to originate in a miasm, at times seems to assume a contagious character. The same may be said of yellow fever. But however this may be, the distinction applies to such diseases as intermittent and remittent fevers, which originating in a miasm, itself springing from the putrescence of animal or vegetable bodies, gives rise to disease which does not reproduce the miasm. Now, between diseases so produced and those arising from contagion properly so called, there is this remarkable distinction: the blood once altered is no longer susceptible of the same contagion, whereas against miasms there is no such security.[51]
In every contagious disease, and perhaps even in those simply arising from miasms, there is an odour which fills the chambers of the sick, and is recognisable at once. Ammonia is very generally present, as it is wherever animal decompositions are going on, that is, putrefaction. The foul airs emanating from stagnant and neglected ditches is composed, as has been long known to chemists, of carbonic acid and sulphuretted hydrogen gases, and these are viewed by some as amongst the most dangerous of miasms. These gases may be destroyed by acid vapours now in common use.[52] From chemistry we also derive another valuable lesson in respect of substances directly destroying human life. The materials ready to undergo putrefaction, and thus to generate miasms, may all be present, and yet no miasms are given out, and man escapes; this security depends upon the absence of that third principle requisite to bring the others into activity.
Thus it happens that in his extensive and elaborate inquiries, Major Tulloch was continually met by difficulties which overthrew at once all existing medical theories, rendering it even probable that the supposed relation of cause and effect between marshes and miasms, and miasm and fever, was merely accidental. In what that third element consists, that immediately exciting power which urges on the decomposition to an extent it had not before attained, rendering that miasm mortal, or at least most dangerous, which heretofore the vital force was able to resist, has not yet been discovered.
Is it electricity? is it ozone?[53] or does it depend on some unknown principle in the elements of the atmosphere, for the detection of which we have no instrument? Does security in such cases depend on the presence in the atmosphere of some such principle as ozone? Whatever be the cause, the fact is certain; epidemics follow cycles of increase and decrease; like comets, they come and disappear at long intervals. Our business in the mean time lies with what is constantly present, in a more or less aggravated form—the malaria continually reproduced, always efficient in certain regions of the earth; in the overcoming of which, as I have endeavoured to show, well-directed human industry is far from unavailing.[54]