18 THE DATA OF BIOLOGY. 



whether 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 dif- 

 ferences 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 substances 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 relation to molecular weight — other 

 things equal, we miist 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 experi- 

 mented, was hydrochloric acid — a compound which is of 

 low molecular weight, is gaseous save under a pressure of 

 forty atmospheres, and ordinarily exists as a liquid, only in 

 combination with water. Again, " hydrate of potash may 

 be said to possess double the velocity of diffusion of sulphate 

 of potash, and sulphate of potash again double the velocity 

 of sugar, alcohol, and sulphate of magnesia," — differences 



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