20 THE DATA OF BIOLOGY. 



One other result of these researches on the relative diffusi- 

 bilities 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 affinities are feeble, will separate on the 

 dialyzer, if their molecular mobilities are strongly con- 

 trasted. Speaking of the hydrochloride of peroxide of iron, 

 he says, " such a compound possesses an element of in- 

 stability in the extremely unequal diffusibility of its con- 

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

 hydrochloric acid gradually diffuses away, leaving the col- 

 loidal 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 which differ 

 widely in their molecular mobilities, though usually so 

 far antagonized by their affinities as not to produce spon- 

 taneous 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- 

 plex compounds composing organic bodies — compounds in 

 which there are various cross affinities leading to a state 

 of chemical tension — this extreme difference in the molecular 

 mobilities must be an important aid to molecular re-arrange- 

 ments. In short, we are here led by concrete evidence to the 



