1844—1849 33 



expressed himself thus: ** Chlorine possesses the singular 

 power of seizing upon the hydrogen in certain substances, and 

 of taking its place atom by atom.'' 



This theory of substitutions was — according to a simple and 

 vivid comparison of Pasteur's — a way of looking upon chemical 

 bodies as upon "molecular edifices, in which one element could 

 be replaced by another without disturbing the structure of the 

 edifice; as if one were to replace, one by one, every stone of a 

 monument by a new stone." Original researches, new and 

 bold ideas, appealed to Pasteur. But his cautious mind pre- 

 vented his boldness from leading him into errors, surprises or 

 hasty conclusions. '*That is possible," he would say, ''but 

 we must look more deeply into the subject." 



When asked by Laurent to assist him with some experiments 

 upon certain theories, Pasteur was delighted at this suggested 

 collaboration, and wrote to his friend Qhappuis : ' ' Even if the 

 work should lead to no results worth publishing, it wiU be 

 most useful to me to do practical work for several months wi^h 

 such an experienced chemist." 



It was partly due to Laurent, that Pasteur entered more 

 deeply into the train of thought which was to lead him to 

 grapple with Mitscherlich 's problem. "One day" (this 

 is a manuscript note of Pasteur's) "one day it happened that 

 M. Laurent — studying, if I mistake not, some tungstate of 

 soda, perfectly crystallized and prepared from the directions of 

 another chemist, whose results he was verifying — showed me 

 through the microscope that this salt, apparently very pure, 

 ^as evidently a mixture of three distinct kinds of crystals, easily 

 recognizable with a little experience of crystalline forms. The 

 lessons of our modest and excellent professor of mineralogy, M. 

 Delafosse, had long since made me love crystallography; so, in 

 order to acquire the habit of using the goniometer, I began to 

 carefuUy study the formations of a very fine series of combina- 

 tions, all very easily crystallized, tartaric acid and the 

 tartrates." He appreciated any favourable influence on his 

 work ; we find in the same note : * ' Another motive urged me to 

 prefer the study of those particular forms. M. de la Provos- 

 taye had just published an almost comi)lete work concerning 

 them; this allowed me to compare as I went along my own 

 observations with those, always so precise, of that clever 

 Bcientist. ' ' 



Pasteur and Laurent's work in common was interrupted. 



