16 LOUIS PASTEUR 



which was then beginning to occupy the attention 

 of chemists was Laurent's especial interest. It had 

 been developed by Dumas, and it appealed strongly 

 to the imagination of Pasteur. According to this 

 theory, chemical changes may occur by the substi- 

 tution of one atom or group of atoms in a molecule 

 by another atom or group of atoms, while the rest 

 of the molecule remains unchanged. Nowadays 

 chemists can build up whole series of allied com- 

 pounds by substituting one element or group for 

 another, thereby building up compounds of greater 

 and greater degrees of complexity. The possibili- 

 ties of orderly constructive chemical transforma- 

 tion which the theory of substitutions suggests 

 were clearly seen by Pasteur who recognized the 

 great importance of this guiding principle in chem- 

 ical research. 



For his doctor's degree he prepared and defended 

 two theses, one in chemistry entitled Researches 

 into the Saturation Capacity of Arsenious Acid. 

 A Study of the Arsenates of Potash, Soda and 

 Ammonia; the other in physics entitled A Study 

 of Phenomena Relative to the Rotary Polarization 

 of Liquids. Both were dedicated to his parents. 

 "Although we cannot judge your essays," wrote the 

 father, "our satisfaction is no less great. But as 



