2 PASTEUR: THE HISTORY OF A MIND 



is he to see justly, that is to say, to judge with the judg- 

 ment of that time? How abstract oneself from what has 

 been learned since, and take on again the necessary 

 ignorance? 



Nevertheless, I shall attempt to do this throughout 

 this volume; but, as it is easy to understand, the greatest 

 difficulties are at the beginning. Familiar as we are 

 to-day with the theories of molecular structure, we have 

 some difficulty in picturing to ourselves the chaotic 

 condition of these ideas among the scientific men of 1840. 



The}' had a knowledge of the chemical molecule. 

 They knew it is formed by a grouping of generally quite 

 stable atoms, the number, weight and nature of which 

 are ordinarily very well defined. They knew, for ex- 

 ample, that there is one atom of chlorine and one of 

 sodium in marine salt, while in calcium carbonate there 

 is one atom of calcium, one atom of carbon, and three 

 atoms of oxygen. They had recognized that the different 

 compound molecules differentiated themselves ordinarily 

 by the number and nature of the atoms composing them; 

 that there are, nevertheless, some which contain the same 

 number of the same atoms without being identical, from 

 which one was led to suppose that they were arranged 

 somewhat differently. But in what did these arrange- 

 ments consist? How do the atoms dispose themselves 

 in relation to each other in a molecule? What is the 

 resultant form for this molecule? These were questions 

 on which no one had clear ideas. 



Crystallography had given no answer, contrary to 

 what we might believe to-day, after the teachings which 

 this science has furnished us. It held to Haiiy's narrow 

 and geometrical conception of the integral crystal mole- 

 cule. We know that he called by this name the little 

 solid, the juxtaposition and superposition of which in an 

 infinite number resulted in the formation of the crystal. 



