FIEST DISCOVERIES. 25 



revealed by the absence of superposability. The result 

 was in great part conformable to those previsions. 

 The substances which acted upon polarised light, as 

 liquids or solutions, were generally found by Pasteur 

 to produce dissymmetric crystals. Some of them, how- 

 ever, notwithstanding their power of crystallisation, 

 exhibited, when crystallised, no dissymmetric face. 

 This difficulty did not deter Pasteur. It gave him, on 

 the contrary, the opportunity of showing that when a 

 theory had in so many cases proved itself correct, an 

 apparent objection must not be assumed insuperable 

 without first sounding it to the bottom. May it not 

 be, he reasoned, that the absence of dissymmetry in 

 substances which have the molecular rotatory power 

 is not an accident ; and may it not be possible, by 

 changing the conditions of the crystallisation, to make 

 the dissymmetry appear ? 



Then, in order to modify the crystalline forms of 

 substances which did not show themselves to be spon- 

 taneously dissymmetrical, Pasteur employed a method 

 which had been often tried before, though its principles 

 could not be explained or its effects foreseen. In 

 imitation of Rome de Lisle, Leblanc, and Beudant, he 

 varied the nature of his solvents ; he introduced into 

 the solution, sometimes an excess of acid or of base, 

 sometimes foreign matters incapable of acting chemi- 

 cally upon those which were to be modified ; he even 

 employed sometimes impure mother liquids. On each 



