22 LOUIS PASTEUR 



of steps, each leading on to the other in a perfectly 

 natural sequence. There are perhaps few better 

 illustrations of the unity of nature and hence the 

 essential oneness of science than that which is 

 afforded by Pasteur's work. 



All along the road traversed by Pasteur we meet 

 with discoveries of capital importance. During his 

 generation Pasteur was the central figure in a great 

 epoch in the history of biology and medicine. His 

 life work was destined to be devoted to revealing 

 the role in nature which is played by the micro- 

 scopic forms of life. If he became a biologist after 

 being a chemist, it was his work in chemistry that 

 determined the living forms on which he worked 

 and the kind of problems which he first endeav- 

 ored to solve. In order to follow Pasteur's early 

 studies it will therefore be desirable to say a few 

 words about crystals which began to arouse his 

 interest in the Ecole Normale. 



Many chemical substances, when they are recov- 

 ered from the liquid in which they are dissolved, 

 assume a perfectly definite crystalline form which 

 is characteristic of their own particular kind of 

 material. Thus common salt crystallizes in cubes, 

 quartz in six sided prisms and the tartrates in eight 

 sided prisms. The angles between the faces or 



