18 THE DATA OF BIOLOGY. 



acid, the true numbers of its formula must be several times 

 greater. It is difficult to avoid associating the inertness of 

 colloids with their high equivalents, particularly where the 

 high number appears to be attained by the repetition of a 

 small number. The inquiry suggests itself whether the col- 

 loid molecule may not be constituted by the grouping 

 together of a number of smaller crystalloid molecules, and 

 w^hether the basis of coUoidality may not really be this com- 

 posite character of the molecule." 



§ 7. A further contrast between colloids and crystalloids, 

 is equally significant in its relations to vital phenomena. 

 Professor Graham points out that the marked differences in 

 volatility displayed by different bodies, are paralleled by 

 differences in the rates of diffusion of different bodies through 

 liquids. As alcohol and ether at ordinary temperatures, and 

 various other substances at higher temperatures, diffuse them- 

 selves in a gaseous form through the air; so a substance in 

 aqueous solution, when placed in contact with a mass of 

 water (in such way as to avoid mixture by circulating currents) 

 diffuses itself through this mass of water. And just as there 

 are various degrees of rapidity in evaporation, so there are 

 various degrees of rapidity in diffusion : " the range also in 

 the degree of diffusive mobility exhibited by different sub- 

 stances appears to be as wide as the scale of vapour-tensions." 

 This parallelism is what might have been looked for ; since 

 the tendency to assume a gaseous state, and the tendency to 

 spread in solution through a liquid, are both consequences of 

 molecular mobility. It also turns out, as was to be expected, 

 that diffusibility, like volatility, has, other things equal, a re- 

 lation to atomic weight — (other things equal, we must say, 

 because molecular mobility must, as pointed out in § 5, be 

 affected by other properties of atoms, besides their inertia). 

 Thus the substance most rapidly diffused of any on which 

 Professor Graham experimented, was hydro-chloric acid — a 

 compound which is of low atomic weight, is gaseous save 



