THE MICROSCOPE. 



"3 



August 23,000 



October 14,000 



November 8,000 



December 7,000 



Heat is not the most important factor in the development of 

 these bodies, but the hygrometric state of the air is. This was to be 

 foreseen as a deduction from the researches made on this subject 

 by Mr. Pasteur. Storms are always followed by an increase in the 

 number of cryptogamic germs; but mineral dust and a few species 

 of microbes, on the contrary, remain fixed to the soil through damp- 

 ness. 



The exact determination of the peculiar vegetations for which 

 the atmosphere serves as a vehicle is made through cultures — the 

 air or water containing the organisms being made to pass into 

 liquors favorable to their nutrition. The liquors used for this pur- 





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pose are the mineral solutions of Pasteur and Cohn, infusion of hay 

 and beet, neutral urine, beef or chicken broth, and Liebig's extract 

 of beef. The difficulty consists in having these solutions absolutely 

 free from all living organisms. Recourse has been had, without 

 success, to ebullition at ioo° — a temperature sufficient to coagulate 

 protoplasm, the physical basis of life, according to Prof. Huxley. 

 Certain spores, however, resist the action of boiling water for several 

 hours. 



Mr. Koch extols the method of discontinuous heating for ster- 

 ilizing liquors. He raises the latter to a temperature lower than 70 

 in order to kill the adult bacteria, and then allows them to cool so as 

 to give the spores time to germinate, and finally raises the tempera- 



