the part of scavenger, merely aids these changes by carrying away the products of decompositions otherwise caused; it equally remains true that these changes are maintained by its instrumentality. Whether the oxygen absorbed and diffused through the system effects a direct oxidation of the organic colloids which it permeates, or whether it first leads to the formation of simpler and more oxidized compounds, which are afterwards further oxidized and reduced to still simpler forms, matters not, in so far as the general result is concerned. In any case it holds good that the substances of which the animal body is built up, enter it in either an unoxidized or in a but slightly oxidized and highly unstable state; while the great mass of them leave it in a fully oxidized and stable state. It follows, therefore, that, whatever the special changes gone through, the general process is a falling from a state of unstable chemical equilibrium to a state of stable chemical equilibrium. Whether this process be direct or indirect, the total molecular re-arrangement and the total motion given out in effecting it, must be the same.
§ 15. There is another species of re-distribution among the component matters of organisms, which is not immediately effected by the affinities of the matters concerned, but is mediately effected by other affinities; and there is reason to think that the re-distribution thus caused is important in amount, if not indeed the most important. In ordinary cases of chemical action, the two or more substances concerned themselves undergo changes of molecular arrangement; and the changes are confined to the substances themselves. But there are other cases in which the chemical action going on does not end with the substances at first concerned, but sets up chemical actions, or changes of molecular arrangement, among surrounding substances that would else have remained quiescent. And there are yet further cases in which mere contact with a substance that is itself quiescent, will cause other substances to undergo rapid metamorphoses. In what we call fermentation, the first species of this communicated chemical action is exemplified. One part of yeast, while itself undergoing molecular change, will convert 100 parts of sugar into alcohol and carbonic acid; and during its own decomposition, one part of diastase "is able to effect the transformation of more than 1000 times its weight of starch into sugar." As illustrations of the second species, may be mentioned those changes which are suddenly produced in many colloids by minute portions of various substances added to them – substances that are not undergoing manifest transformations, and suffer no appreciable effects from the contact. The nature of the first of these two kinds of communicated molecular change, which here chiefly concerns us, may be rudely represented by certain visible changes communicated from mass to mass, when a series of masses has been arranged in a special way. The simplest example is that furnished by the child's play of setting bricks on end in a row, in such positions that when the first is overthrown it overthrows the second, the second the third, the third the fourth, and so on to the end of the row. Here we have a number of units severally placed in unstable equilibrium, and in such relative positions that each, while falling into a state of stable equilibrium, gives an impulse to the next sufficient to make the next, also, fall from unstable to stable equilibrium. Now since, among mingled compound molecules, no one can undergo change in the arrangement of its parts without a molecular motion that must cause some disturbance all round; and since an adjacent molecule disturbed by this communicated motion, may have the arrangement of its constituent atoms altered, if it is not a stable arrangement; and since we know, both that the molecules which are changed by this so-called catalysis are unstable, and that the molecules resulting from their changes are more stable; it seems probable that the transformation is really analogous, in principle, to the familiar one named. Whether thus interpretable or not, however, there is good reason for thinking that to this kind of action is due a large amount of vital metamorphosis. Let us contemplate the several groups of facts which point to this conclusion.9
In the last chapter (§ 2) we incidentally noted the extreme instability of nitrogenous compounds in general. We saw that sundry of them are liable to explode on the slightest incentive – sometimes without any apparent cause; and that of the rest, the great majority are very easily decomposed by heat, and by various substances. We shall perceive much significance in this general characteristic when we join it with the fact that the substances capable of setting up extensive molecular changes in the way above described are all nitrogenous ones. Yeast consists of vegetal cells containing nitrogen, – cells that grow by assimilating the nitrogenous matter contained in wort. Similarly, the "vinegar-plant," which greatly facilitates the formation of acetic acid from alcohol, is a fungoid growth that is doubtless, like others of its class, rich in nitrogenous compounds. Diastase, by which the transformation of starch into sugar is effected during the process of malting, is also a nitrogenous body. So too is a substance called synaptase – an albumenous principle contained in almonds, which has the power of working several metamorphoses in the matters associated with it. These nitrogenized compounds, like the rest of their family, are remarkable for the rapidity with which they decompose; and the extensive changes produced by them in the accompanying carbo-hydrates, are found to vary in their kinds according as the decompositions of the ferments vary in their stages. We have next to note, as having here a meaning for us, the chemical contrasts between those organisms which carry on their functions by the help of external forces, and those which carry on their functions by forces evolved from within. If we compare animals and plants, we see that whereas plants, characterized as a class by containing but little nitrogen, are dependent on the solar rays for their vital activities; animals, the vital activities of which are not thus dependent, mainly consist of nitrogenous substances. There is one marked exception to this broad distinction, however; and this exception is specially instructive. Among plants there is a considerable group – the Fungi – many members of which, if not all, can live and grow in the dark; and it is their peculiarity that they are very much more nitrogenous than other plants. Yet a third class of facts of like significance is disclosed when we compare different portions of the same organism. The seed of a plant contains nitrogenous substance in a far higher ratio than the rest of the plant; and the seed differs from the rest of the plant in its ability to initiate, in the absence of light, extensive vital changes – the changes constituting germination. Similarly in the bodies of animals, those parts which carry on active functions are nitrogenous; while parts that are non-nitrogenous – as the deposits of fat – carry on no active functions. And we even find that the appearance of non-nitrogenous matter throughout tissues normally composed almost wholly of nitrogenous matter, is accompanied by loss of activity: what is called fatty degeneration being the concomitant of failing vitality. One more fact, which serves to make still clearer the meaning of the foregoing ones, remains – the fact, namely, that in no part of any organism where vital changes are going on, is nitrogenous matter wholly absent. It is common to speak of plants – or at least all parts of plants but the seeds – as non-nitrogenous. But they are only relatively so; not absolutely. The quantity of albumenoid substance in the tissues of plants, is extremely small compared with the quantity contained in the tissues of animals; but all plant-tissues which are discharging active functions have some albumenoid substance. In every living vegetal cell there is a certain part that includes nitrogen as a component. This part initiates those changes which constitute the development of the cell. And if it cannot be said that it is the worker of all subsequent changes undergone by the cell, it nevertheless continues to be the part in which the independent activity is most marked.
Looking at the evidence thus brought together, do we not get an insight into the actions of nitrogenous matter as a worker of organic changes? We see that nitrogenous compounds in general are extremely prone to decompose: their decomposition often involving a sudden and great evolution of energy. We see that the substances classed as ferments, which, during their own molecular changes, set up molecular changes in the accompanying carbo-hydrates, are all nitrogenous. We see that among classes of organisms, and among the parts of each organism, there is a relation between the amount of nitrogenous matter present and the amount of independent activity. And we see that even in organisms and parts of organisms where the activity is least, such changes as do take place are initiated by a substance containing nitrogen. Does it not seem probable, then, that these extremely unstable compounds have everywhere the effect of communicating to the less unstable compounds associated with them, molecular movements towards a stable state, like those they are themselves undergoing? The changes which we thus suppose nitrogenous matter to produce in the body, are clearly analogous to
On now returning to the subject after many years, I meet with some evidence recently assigned, in a paper read before the Royal Society by Mr. J. W. Pickering, D.Sc. (detailing results harmonizing with those obtained by Prof. Grimaux), showing clearly how important an agent in vital actions is this production of isomeric changes by slight changes of conditions. Certain artificially produced substances, simulating proteids in other of their characters and reactions, were found to simulate them in coagulability by trifling disturbances. "In the presence of a
