A System of Logic, Ratiocinative and Inductive (Vol. 1 of 2)

by John Stuart Mill

Here, therefore, is a law of nature of great simplicity, but which, owing to the extremely special and limited character of the phenomena in which alone it can be detected experimentally, (because in them alone its results are not intermixed and blended with those of other laws,) had been very little recognised by chemists, and no one could have ventured, on experimental evidence, to affirm it as a law common to all chemical action; owing to the impossibility of a rigorous employment of the Method of Difference where the properties of different kinds of substance are involved, an [pg 489] impossibility which we noticed and characterized in a previous chapter.[90] Now this extremely special and apparently precarious generalization has, in the hands of Liebig, been converted, by a masterly employment of the Deductive Method, into a law pervading all nature, in the same way as gravitation assumed that character in the hands of Newton; and has been found to explain, in the most unexpected manner, numerous detached generalizations of a more limited kind, reducing the phenomena concerned in those generalizations into mere cases of itself.

The contagious influence of chemical action is not a powerful force, and is only capable of overcoming weak affinities: we, may, therefore, expect to find it principally exemplified in the decomposition of substances which are held together by weak chemical forces. Now the force which holds a compound substance together is generally weaker, the more compound the substance is; and organic products are the most compound substances known, those which have the most complex atomic constitution. It is, therefore, upon such substances that the self-propagating power of chemical action is likely to exert itself in the most marked manner. Accordingly, first, it explains the remarkable laws of fermentation, and some of those of putrefaction. “A little leaven,” that is, dough in a certain state of chemical action, impresses a similar chemical action upon “the whole lump.” The contact of any decaying substance, occasions the decay of matter previously sound. Again, yeast is a substance actually in a process of decomposition from the action of air and water, evolving carbonic acid gas. Sugar is a substance which, from the complexity of its composition, has no great energy of coherence in its existing form, and is capable of being easily converted (by combination with the elements of water) into carbonic acid and alcohol. Now the mere presence of yeast, the mere proximity of a substance of which the elements are separating from each other, and combining with the elements of water, causes sugar to undergo the same change, [pg 490] giving out carbonic acid gas, and becoming alcohol. It is not the elements contained in the yeast which do this. “An aqueous infusion of yeast may be mixed with a solution of sugar, and preserved in vessels from which the air is excluded, without either experiencing the slightest change.” Neither does the insoluble residue of the yeast, after being treated with water, possess the power of exciting fermentation. (Here we have the method of Difference). It is not the yeast itself, therefore; it is the yeast in a state of decomposition. The sugar, which would not decompose and oxidize by the mere presence of oxygen and water, is induced to do so when another oxidation is at work in the midst of it.

By the same principle Liebig is enabled to explain many cases of malaria; the pernicious influence of putrid substances; a variety of poisons; contagious diseases; and other phenomena. Of all substances, those composing the animal body are the most complex in their composition, and are in the least stable condition of union. The blood, in particular, is the most unstable compound known. It is, therefore, not surprising that gaseous or other substances, in the act of undergoing the chemical changes which constitute, for instance, putrefaction, should, when brought into contact with the tissues by respiration or otherwise, and still more when introduced by inoculation into the blood itself, impress upon some of the particles a chemical action similar to its own; which is propagated in like manner to other particles, until the whole system is placed in a state of chemical action more or less inconsistent with the chemical conditions of vitality.

Of the three modes in which we observed in the last chapter that the resolution of a special law into more general ones may take place, this speculation exemplifies the second. The laws explained are such as this, that yeast puts sugar into a state of fermentation. Between the remote cause, the presence of yeast, and the consequent fermentation of the sugar, there has been interpolated a proximate cause, the chemical action between the particles of the yeast and the elements of air and water. The special law is thus resolved into two others, more general than itself: the first, that yeast [pg 491] is decomposed by the presence of air and water; the second, that matter undergoing chemical action has a tendency to produce similar chemical action in other matter in contact with it. But while the investigation thus aptly exhibits the second mode of the resolution of a complex law, it no less happily exemplifies the third; the subsumption of special laws under a more general law, by gathering them up into one more comprehensive expression which includes them all. For the curious fact of the contagious nature of chemical action is only raised into a law of all chemical action by these very investigations; just as the Newtonian attraction was only recognised as a law of all matter when it was found to explain the phenomena of terrestrial gravity. Previously to Liebig's investigations, the property in question had only been observed in a few special cases of chemical action; but when his deductive reasonings have established that innumerable effects produced upon weak compounds, by substances none of whose known peculiarities would account for their having such a power, might be explained by considering the supposed special property to exist in all those cases, these numerous generalizations on separate substances are brought together into one law of chemical action in general: the peculiarities of the various substances being, in fact, eliminated, just as the Newtonian deduction eliminated from the instances of terrestrial gravity the circumstance of proximity to the earth.

§ 2. Another speculation of the same chemist, which, if it should ultimately be found to agree with all the facts of the extremely complicated phenomenon to which it relates, will constitute one of the finest examples of the Deductive Method on record, is his theory of respiration.

The facts of respiration, or in other words the special laws which it is attempted to explain from, and resolve into, more general ones, are, that the blood in passing through the lungs absorbs oxygen and gives out carbonic acid gas, changing thereby its colour from a blackish purple to a brilliant red. The absorption and exhalation are evidently chemical [pg 492] phenomena; and the carbon of the carbonic acid must have been derived from the body, that is, must have been absorbed by the blood from the substances with which it came into contact in its passage through the organism. Required to find the intermediate links—the precise nature of the two chemical actions which take place; first, the absorption of the carbon or of the carbonic acid by the blood, in its circulation through the body; next, the excretion of the carbon, or the exchange of the carbonic acid for oxygen, in its passage through the lungs.

Dr. Liebig believes himself to have found the solution of this vexata quæstio in a class of chemical actions in which scarcely any less acute and penetrating inquirer would have thought of looking for it.

Blood is composed of two parts, the serum and the globules. The serum absorbs and holds in solution carbonic acid in great quantity, but has no tendency either to part with it or to absorb oxygen. The globules, therefore, are concluded to be the portion of the blood which is operative in respiration. These globules contain a certain quantity of iron, which from chemical tests is inferred to be in the state of oxide.

Dr. Liebig recognised, in the known chemical properties of the oxides of iron, laws which, if followed out deductively, would lead to the prediction of the precise series of phenomena which respiration exhibits.

There are two oxides of iron, a protoxide and a peroxide. In the arterial blood the iron is in the form of peroxide: in the venous blood we have no direct evidence which of the oxides is present, but the considerations to be presently stated lead to the conclusion that it is the protoxide. As arterial and venous blood are in a perpetual state of alternate conversion into one another, the question arises, in what circumstances the protoxide of iron is capable of being converted into the peroxide, and vice versâ. Now the protoxide readily combines with oxygen in the presence of water, forming the hydrated peroxide: these conditions it finds in passing through the lungs; it derives oxygen [pg 493] from the air, and finds water in the blood itself. This would already explain one portion of the phenomena of respiration. But the arterial blood, in quitting the lungs, is charged with hydrated peroxide: in what manner is the peroxide brought back to its former state?