MICROSCOPIC LIFE IN THE AIR. 3J 



ocls be devised are the ones that are still used to detect the presence 

 of these plants in the atmosphere. Generally, these methods are 

 founded on the fact that organic liquids become peopled with micro- 

 phytes, or remain unchanged, according as the air comes to them 

 charged with its normal quantity of germs, or after having been 

 cleared of them by filtration. We can, therefore, examine the bac- 

 teria in the atmosphere by causing the air or the rain-water containing 

 them to pass into liquors favorable to their nutrition, but previously 

 free from them. Liquids for the culture of bacteria are easy to pro- 

 cure. Among them are the mineral solutions of Pasteur and of Cohn, 

 infusions of hay and of turnips, neutral urine, broths of chicken or 

 beef, and Liebig's extract. It is, however, very hard to obtain such 

 liquids absolutely pure of every living being. Eminent physiologists 

 have thought that all the germs could be killed by boiling them for a 

 considerable time. Apparently the protoplasm, being an albuminoid 

 substance, would be coagulated at a temperature of between 167 and 

 176 ; but very exact experiments have shown that while the proto- 

 plasms of different living beings belong to the same class of sub- 

 stances, they are not identical. Marked differences in this respect 

 have been perceived even in the same beings. Thus, in the bacillus 

 the protoplasm of the developed organism and that of the spore are 

 not of the same quality. The former is in active life, the second in a 

 state of life so low that it appears latent. A spore of this kind, as M. 

 Chamberland has observed, will resist boiling water for hours, while 

 the batonnet which is developed from it would perish rapidly in the 

 same water at 122. 



M. Koch has conceived a method of discontinuous heating to ster- 

 ilize liquids that are coagulable by heat. He raises his liquid to a 

 temperature of not quite 158 to kill the adult bacteria ; then having 

 cooled it, to give the spores time to germinate, he raises it again to 

 about 158 ; and he believes that he can in this way destroy all the 

 germs. M. Miquel makes a just criticism of this singular theory. 

 We arrange that the spores of the microbes " must germinate in twelve 

 or twenty-four hours, so that we may surely kill them if they go into 

 the trap we set for them. But some of the germs may be obstinate 

 or hardy, and we make a new trial, and for prudence a third and a 

 fourth trial, after which we assume that all the bacteria have been 

 destroyed. Unfortunately for the method of discontinuous heating, 

 there are wary germs the development of which does not begin till 

 after the fifth, the tenth, and even the twentieth day, and which, far 

 from being stimulated in their growth by the successive heatings, 

 at every repetition shut themselves up more closely in their latent 

 seed-life. This method of sterilization can not, then, be depended 

 upon." 



A still more subtile cause of error must be guarded against. Cohn's 

 mineral liquid will remain clear for an indefinite period after having 



TOL. XXIII. 3 



