20 THE DATA OF BIOLOGY. 



elapsed." So that we must regard the colloidal compounds 

 of which organisms are built, as having by their physical 

 nature, the ability to separate colloids from crystalloids, and 

 to let the crystalloids pass through them with scarcely any 

 resistance. 



One other result of these researches on the relative diffu- 

 sibilities of different substances, has a meaning for us. Pro- 

 fessor Graham finds, that not only does there take place by 

 dialysis, a separation of mixed substances which are unlike in 

 their molecular mobilities ; but also that combined substances 

 between which the affinity is feeble, will separate on the 

 dialyzer, if their molecular mobilities are strongly con- 

 trasted. Speaking of the hydro- chlorate of peroxide of 

 iron, he says, " such a compound possesses an element of 

 instability in the extremely unequal diffusibility of its 

 constituents ; " and he points out that when dialyzed, the 

 hydro-chloric acid gradually diffuses away, leaving the 

 colloidal peroxide of iron behind. Similarly, he remarks of 

 the peracetate of iron, that it "may be made a source of 

 soluble peroxide, as the salt referred to is itself decomposed 

 to a great extent by diffusion on the dialyzer." Now this 

 tendency to separate displayed by substances that differ 

 widely in their molecular mobilities, though usually so 

 far antagonized by their affinities as not to produce sponta- 

 neous decomposition, must, in all cases, induce a certain 

 readiness to change which would not else exist. The un- 

 equal mobilities of the combined atoms, must give disturbing 

 forces a greater power to work transformations than they 

 would otherwise have. Hence the probable significance of a 

 fact named at the outset, that while three of the chief organic 

 elements have the greatest atomic mobilities of any elements 

 known, the fourth, carbon, has the least atomic mobility of 

 known elements. Though, in its simple compounds, the 

 affinities of carbon for the rest are strong enough to prevent 

 the effects of this great difference from clearly showing them- 

 selves ; yet there seems reason to think, that in those com- 



