12 THE DATA OF BIOLOGY. 



exist as such in organic matter. These two classes of sub- 

 stances exhibit in different degrees, the properties to which 

 we have been directing our attention. The lower alcohols, 

 their allies and derivatives, which possess greater molecular 

 mobility and chemical stability than the rest of these ternary 

 compounds, are not found in animal or vegetal bodies. While 

 the sugars and amylaceous substances, the fixed oils and fats, 

 the gums and resins, which have all of them much less mole- 

 cular mobility, and are, chemically considered, 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 

 two groups are contrasted in their properties in the same way 

 as are the parallel groups of ternary 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 



