4 PASTEUR: THE HISTORY OF A MIND 



shortly to change ideas on this point. By showing that 

 without changing the form of a crystal of calcium car- 

 bonate, Iceland spar for example, it was possible to re- 

 place as large a number as one wished of atoms of 

 calcium by an equal number of atoms of magnesium, 

 Mitscherlich introduced, in a form still vague, a struc- 

 tural conception of the crystal entirely different from 

 that of Haiiy. If atoms of calcium and magnesium can, 

 without any change of form, be substituted one for the 

 other in a crystal, it is because they are of the same form, 

 or what amounts to the same thing, because they act 

 at a distance in the same way. Thus the geometry of 

 the integral molecule was abandoned to approach the 

 geometry of the chemical molecule, and one could say 

 that calcium, magnesium, iron, manganese, and zinc, 

 which give carbonates crystallizing in the same form as 

 Iceland spar, have atoms of the same form, while baryum 

 and strontium, which give entirely different carbonates, 

 not isomorphic with the first, have atoms of another form. 

 As in the molecules of the carbonates of calcium, of iron, 

 of magnesium, of manganese and of zinc everything is 

 identical (except the metals, whose atoms are of the same 

 form), it was assumed that the chemical molecules 

 of these different bodies are also of the same form, and 

 by conceiving that the integral molecules of the different 

 crystals are also of the same form, the conclusion was 

 reached that between the crystalline form of any sub- 

 stance whatsoever and the constitution of its chemical 

 molecule a relation existed which, though still vague, was 

 most certainly much closer than the theory of Haiiy 

 assumed it to be. 



Clearly not all of these deductions were based on solid 

 foundations, and one could almost as well have explained 

 these new facts on the doctrine of Haiiy by admitting 

 that the chemical molecules of different forms could 



