The Evolution of the Sciences 



exist in the air and invade with it the liquid 

 which they fertilise. 



Contagions and epidemics spread in the 

 mineral world by a similar process. Railway 

 engineers frequently find that all the rails in 

 the same region have become brittle and re- 

 quire replacing at the same time. A few years 

 ago, it was observed that the tin buttons of 

 certain Russian regiments were all becoming 

 brittle and breaking up into thousands of 

 grains. This disease of the buttons was shown 

 to be distinctly epidemic. If a uniform of one 

 of the contaminated regiments was brought 

 to an unaffected district, numerous cases of 

 the disease occurred in its vicinity. 



The explanation of this fact is simple. Tin 

 can be transformed by the severe cold of winter 

 into an extremely crystalline variety gray tin; 

 but this transformation can occur at a higher 

 temperature in contact with an existing crystal. 

 Thus the buttons attacked produce, by their 

 pulverisation, thousands of crystalline germs, 

 which give rise all around them to the same 

 modification. 



But it is not crystals alone that imitate the 



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