m MEDICAL SCIENCE. 13 



tion at a very open angle of intersection. I think we 

 may find an analogy for it in electrical induction, the 

 disturbance of the equilibrium of the electricity of a 

 body by the approach of a charged body to it, without 

 interchange of electrical conditions between the two 

 bodies. But an analogy is not an explanation, and 

 why a few drops of yeast should change a saccharine 

 mixture to carbonic acid and alcohol, — a little leaven 

 leavening the whole lump, — not by combining with it, 

 but by setting a movement at work, we not only can- 

 not explain, but the fact is such an exception to the 

 recognized laws of combination, that Liebig is unwilling 

 to admit the new force at all to which BerzeHus had 

 given the name so generally accepted. 



The phenomena of isomerism, or identity of compo- 

 sition and proportions of constituents with difference 

 of qualities, and of isomorphism, or identity of form in 

 crystals which have one element substituted for another, 

 were equally surprises to science ; and although the 

 mechanism by which they are brought about can be 

 to a certain extent explained by a reference to the 

 hypothetical atoms of which the elements are consti- 

 tuted, yet this is only turning the difficulty into a frac- 

 tion with an infinitesimal denominator and an infinite 

 numerator. 



So far we have studied the working of force and 

 its seeming anomalies in purely chemical phenomena* 

 But we soon find that chemical force is developed by 

 various other physical agencies, — by heat, by light, by 



