10 THE DATA OF BIOLOGY. 



The like is still more conspicuous in other groups of the 

 hydro-carbons, as in the essential oils : sixteen to twenty of 

 which are severally isomeric with essential oil of turpentine. 

 Here the particular kind of molecular mobility implied by 

 these metamorphoses, is well shown : essential oil of turpen- 

 tine being converted into a mixture of several of these poly- 

 merides, by simple exposure to a heat of 460°. 



There is one further fact respecting these binary compounds 

 of the four chief organic elements, which must not be over- 

 looked. Those of them which form parts of the living tissues 

 of plants and animals (excluding water which has a me- 

 chanical function, and carbonic acid which is a product of 

 decomposition) are confined to one group — the hydro-carbons. 

 And of this group, which is on the average characterized by 

 comparative instability and inertness, these hydro-carbons 

 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 accom- 

 panies their increased complexity, there is^ on the average, 

 a further marked decrease of molecular mobility. Scarcely any 

 of them maintain a gaseous state of 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 de- 

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

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

 And throughout the whole series 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 



