24 PASTEUR 



state of this science at the moment when he be- 

 gan his work. 



In 1840 the men of science had only chaotic 

 knowledge of the molecular structure of crys- 

 tals. "They knew the chemical molecule/' 

 writes M. Duclaux, the great authority who was 

 one of Pasteur's disciples; "they knew that it 

 is formed of an ordinarily fairly stable group 

 of atoms, of which the number, the weight and 

 the nature may usually be clearly defined. 

 They knew, for example, that there are one 

 atom of chlorine and one atom of sodium in sea 

 salt, one atom of calcium, one atom of carbon 

 and three atoms of oxygen in carbonate of lime. 

 They had recognised that different composite 

 molecules are ordinarily differentiated by the 

 number and nature of their component atoms, 

 but that nevertheless there are some which con- 

 tain the same number of the same atoms with- 

 out for that reason being identical, so that they 

 were led to suspect that they differed in the ar- 

 rangement of their atoms. What could be the 

 relative disposition of these atoms, one to an- 



