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.