In view of the extraordinary significance which we must assign to the plasm—as the universal vehicle of all the vital phenomena (or "the physical basis of life," as Huxley said)—it is very important to understand clearly all its properties, especially the chemical ones. This is rendered somewhat difficult from the circumstance that the plasm is, in most of the organic cells, closely bound up with other substances—the various plasma products; it can rarely be isolated in its purity, and can never be had pure in any quantity. Hence we are for the most part dependent on the imperfect, and often ambiguous, results of microscopic and microchemical research.
In every case where we have with great difficulty succeeded in examining the plasm as far as possible and separating it from the plasma-products, it has the appearance of a colorless, viscous substance, the chief physical property of which is its peculiar thickness and consistency. The physicist distinguishes three conditions of inorganic matter—solid, fluid, and gaseous. Active living protoplasm cannot strictly be described as either fluid or solid in the physical sense. It presents an intermediate stage between the two which is best described as viscous; it is best compared to a cold jelly or solution of glue. Just as we find the latter substance in all stages between the solid and the fluid, so we find in the case of protoplasm. The cause of this softness is the quantity of water contained in the living matter, which generally amounts to a half of its volume and weight. The water is distributed between the plasma molecules, or the ultimate particles of living matter, in much the same way as it is in the crystals of salts, but with the important difference that it is very variable in quantity in the plasm. On this depends the capacity for absorption or imbibition in the plasm, and the mobility of its molecules, which is very important for the performance of the vital actions. However, this capacity of absorption has definite limits in each variety of plasm; living plasm is not soluble in water, but absolutely resists the penetration of any water beyond this limit.
The chemistry of living matter is the most important and interesting, but at the same time the most difficult and obscure, part of the whole of biological chemistry. In spite of the innumerable and careful investigations which have been made of it by the ablest physiologists and chemists in the second half of the nineteenth century, we are still far from a satisfactory solution of this fundamental problem of biology. This is due partly to the extraordinary difficulty of isolating pure living plasm and subjecting it to chemical analysis, and partly to the many errors and misunderstandings that have arisen through one-sided treatment of the subject, and especially through confusion of the chemical and morphological features of plasm. We can thus understand the contradictory views that are still put forward by distinguished chemists and physiologists, zoologists and botanists. As I cannot deal here with the very extensive, elaborate, and contradictory literature of the subject, I must be content to give a brief summary of the conclusions I have reached by my reading and my own studies of plasm (begun in 1859).
To begin with, we must clearly understand that protoplasm—in the most general sense in which we here take it—is a chemical substance, not a "mixture of different substances," or a "mixture of a small quantity of solid matter with a good deal of fluid." As Richard Neumeister very well observes: "We seek the nature of protoplasm in the peculiar processes which take place in its constituent matter. Protoplasm is for us a chemical matter, so pronounced, in fact, that the highest chemical actions that we know of are embodied in it." I must, from my point of view, entirely reject Oscar Hertwig's conception of living matter as a "mixture" of a number of chemical elements; because chemistry applies this phrase to various gases and powdery substances which are completely indifferent to each other—a property which we certainly do not find in the constituents of protoplasm. When we speak of the living matter or protoplasm, the general phrase does not imply that the substance may not have a distinctive composition in each particular case. And when we find many biologists still conceiving protoplasm as a mixture of various substances, the error is generally due to a confusion of the chemical idea with the morphological, and to a belief that certain structural features of the plasm are primary, whereas they are only secondary, products of the vital process itself in the cell-body.
The older biologists who first introduced the name protoplasm and studied it carefully recognized that this living matter belonged to the albuminous (or proteid) group. The many characteristics which distinguish these nitrogenous carbon-compounds from all other chemical compounds—their behavior towards acids and bases, their peculiar color-reaction towards certain salts, their decomposition-products, etc.—are found in all the plasma-substances, and in all the other albuminoids. This is quite in agreement with the results of quantitative analysis. However differently the various plasma-substances behave in detail, they always exhibit the same general composition as the other albuminoids out of the five "organogenetic elements"—namely, in point of weight, fifty-one to fifty-four per cent. carbon, twenty-one to twenty-three per cent. oxygen, fifteen to seventeen per cent. nitrogen, six to seven per cent. hydrogen, and one to two per cent. sulphur. However, there is a good deal of variety and complication in the way in which the atoms of these five elements are combined in albumin and their molecules are grouped. Hence the question of the chemical nature of the plasma-substances compels us now to look for a moment at the larger group of albuminoids to which they belong.
The carbon-compounds which we comprise under the chemical title of the albumins or proteids are the most remarkable, but also, unfortunately, the least known, of all bodies. The attempt to examine them closely encounters extraordinary difficulties, greater than in any other group of chemical compounds. Everybody is familiar with the appearance of ordinary albumin, from the transparent viscous albumin that surrounds the yolk in the hen's egg, and which becomes a white, opaque, and solid mass when it is cooked. However, this special form of albumin, which we can get so easily in any quantity from the eggs of birds and reptiles, is only one of the innumerable kinds of albumin, or species of protein, that are to be found in the bodies of the various animals and plants. Chemists have hitherto tried in vain to master the chemical structure of these obscure protein-compounds. They are only rarely to be found in chemically pure form as crystals. As a rule, they are in the colloid form, or uncrystallized jelly-like masses, which offer a much greater resistance than crystals to the passage through a porous medium by diosmosis (see p. 39). However, although we have not yet succeeded in penetrating the molecular constitution of the albumins, the laborious research of chemists has yielded some general results which are of great importance for our purpose. We have, in the first place, a general idea of their molecular constitution.
Molecules are the smallest homogeneous parts into which a body can be divided without altering its chemical character. Hence the molecules of every chemical compound are made up of two or more atoms of different kinds. The greater the number of atoms in each compound, the higher is its molecular weight. The space between the molecules and their component atoms is filled with imponderable and highly elastic ether. As even the largest molecules occupy only a very tiny space, and remain far below the range of the most powerful microscope, all our ideas of their composition depend on general physical theories and special chemical hypotheses. Nevertheless, stereochemistry, the modern science of the molecular structure of chemical compounds, is not only a perfectly legitimate section of natural philosophy, but it yields the most important conclusions as to the mutual attractions of the elements and the invisible movements of the atoms in combining. It further enables us to calculate approximately the relative size of the molecules and the number of atoms that are grouped together in them. However, the albuminoids present the greatest difficulty of all in this calculation, and their structural features are still very obscure. Nevertheless, science has reached certain general conclusions, which we may formulate in the following propositions:
1. The molecule of albumin is unusually large, and therefore its molecular weight is very high (higher than in most or all other compounds).
2. The number of atoms composing it is very large (probably much more than a thousand).
3. The disposition of the atoms and groups of atoms in the albuminous molecule is very complicated, and at the same time very unstable—that is to say, very changeable and easily altered.