10 THE DATA OF BIOLOGY. 



carbo-hydrates.* And of this group, which is on the average 

 characterized by comparative instability and inertness, these 

 carbo-hydrates found in living tissues are among the most 

 unstable and inert. 



3. Passing now to the substances which contain three of 

 these chief organic elements, we have first to note that along 

 with the greater atomic weight which mostly accompanies 

 their increased complexity, there is, on the average, a further 

 marked decrease of molecular mobility. Scarcely any of 

 them maintain a gaseous state at ordinary temperatures. 

 One class of them only, the alcohols and their derivatives, 

 evaporate under the usual atmospheric pressure; but not 

 rapidly unless heated. The fixed oils, though they show 

 that molecular mobility implied by an habitually liquid state, 

 show this in a lower degree than the alcoholic compounds; 

 and they cannot be reduced to the gaseous state without 

 decomposition. In their allies, the fats, which are solid unless 

 heated, the loss of molecular mobility is still more marked. 

 And throughout the whole scries of the fatty acids, in which 

 to a fixed proportion of oxygen there are successively added 

 higher equimultiples of carbon and hydrogen, we see how the 

 molecular mobility decreases with the increasing sizes of the 

 molecules. In the amylaceous and sugar-group of com- 

 pounds, solidity is the habitual state: such of them as can 

 assume the liquid form, doing so only when heated to 300 

 or 400 F. ; and decomposing when further heated, rather 

 than become gaseous. Resins and gums exhibit general 

 physical properties of like character and meaning. 



In chemical stability these triatomic compounds, consid- 

 ered as a group, are in a marked degree below the diatomic 

 ones. The various sugars and kindred bodies, decompose at no 



* The name hydro-carbons was here used when these pages were written, 

 thirty-four years ago. It was the name then current. In this case, as in 

 multitudinous other cases, the substitution of newer words and phrases for 

 older ones, is somewhat misleading. Putting the thoughts of 1862 in the 

 language of 1897 gives an illusive impression of recency. 



