12 THE DATA OF BIOLOGY. 



them much less molecular mobility, and are, chemically con- 

 sidered, more unstable and inert, are components of the 

 living tissues of plants and animals. 



§ 4. Among compounds containing all the four chief 

 organic elements, a division analogous to that just named 

 may be made. There are some which result from the decom- 

 position of living tissues; there are others which make parts 

 of living tissues in their state of integrity; and these tv/o 

 groups are contrasted in their properties in the same way as 

 are the parallel groups of triatomic compounds. 



Of the first division, certain products found in the animal 

 excretions are the most important, and the only ones that 

 need be noted; such, namely, as urea, kreatine, kreatinine. 

 These animal-bases exhibit much less molecular mobility than 

 the average of the substances treated of in the last section: 

 being solid at ordinary temperatures, fusing, where fusible at 

 all, at temperatures above that of boiling water, and having 

 no power to assume a gaseous state. Chemically considered, 

 their stability is low, and their activity but small, in com- 

 parison with the stabilities and activities of the simpler com- 

 pounds. 



It is, however, the nitrogenous constituents of living tis- 

 sues, that display most markedly those characteristics of 

 which we have been tracing the growth. Albumen, fibrin, 

 casein, and their allies, are bodies in which that molecular 

 mobility exhibited by three of their components in so high a 

 degree is reduced to a minimum. These substances are known 

 only in the solid state. That is to say, when deprived of the 

 water usually mixed with them, they do not admit of fusion, 

 much less of volatilization To which add, that they have not 

 even that molecular mobility which solution in water implies ; 

 since, though they form viscid mixtures with water, they do 

 not dissolve in the same perfect way as do inorganic com- 

 pounds. The chemical characteristics of these sub- 

 stances are instability and inertness carried to the extreme. 

 How rapidly albumenoid matters decompose under ordinary 



