4 THE DATA OF BIOLOGY. 



substance is its degree of molecular mobility; and its degree 

 of molecular mobility more or less sensibly affects the molec- 

 ular mobilities of the various compounds into which it enters. 

 Hence we may infer some relation between the gaseous form 

 of three out of the four chief organic elements, and that 

 comparative readiness displayed by organic matters to un- 

 dergo those changes in the arrangement of parts which we 

 call development, and those transformations of motion which 

 we call function. 



Considering them chemically instead of physically, it is to 

 be remarked that three out of these four main components 

 of organic matter, have affinities which are narrow in their 

 range and low in their intensity. Hydrogen, it is true, may 

 be made to combine with a considerable number of other ele- 

 ments ; but the chemical energy which it shows is scarcely at 

 all shown within the limits of the organic temperatures. Of 

 carbon it may similarly be said that it is totally inert at ordi- 

 nary heats; that the number of substances with which it 

 unites is not great; and that in most cases its tendency to 

 unite with them is but feeble. Lastly, this chemical indiffer- 

 ence is shown in the highest degree by nitrogen — an element 

 which, as we shall hereafter see, plays the leading part in 

 organic changes. 



Among the organic elements (including under the title 

 not only the four chief ones, but also the less conspicuous re- 

 mainder), that capability of assuming different states called 

 allotropism, is frequent. Carbon presents itself in the three 

 unlike conditions of diamond, graphite, and charcoal. Under 

 certain circumstances, oxygen takes on the form in which it 

 is called ozone. Sulphur and phosphorus (both, in small 

 proportions, essential constituents of organic matter) have 

 allotropic modifications. Silicon, too, is allotropic ; while its 

 oxide, silica, which is an indispensable constituent of many 

 lower organisms, exhibits the analogue of allotropism — iso- 

 merism. Xo other interpretation being possible we are 

 obliged to regard allotropic change as some change of molecular 



