6 POPULAR SCIENCE. 



paste, enclosed under a glass bell and exposed to air thus 

 treated, soon became pervaded with its family of animalcules ; 

 and similarly in respect to other bodies in which the pheno- 

 mena of organic life would under ordinary circumstances be 

 seen. In trying other forms of air-purification, however, 

 cotton-wool pressed hard was at length alighted upon as 

 being wholly efficient. More than one experimenter proved 

 that certain things naturally prone to decompose suffered no 

 decomposition when exposed to air that had been carefully 

 filtered through cotton-wool. The inference was inevitable : 

 the cotton-wool must have filtered away the floating germs 

 of organic life. 



There was no spontaneous generation. In the cotton- 

 wool these things must be, if one could only discover them. 

 But how 1 how reveal and make manifest those microscopic 

 forms of life ? The task might well have seemed hopeless ; 

 but it has been accomplished. Proof having been made, as 

 already stated, that filtration of atmospheric air through a 

 pledget of cotton-wool effectually separated the organic germs 

 naturally pervading the atmosphere, a French experimenter 

 hit upon the following ingenious modification of the experi- 

 ment. Instead of using ordinary cotton-wool, he employed 

 gun-cotton wool, which, not differing from ordinary cotton 

 in texture, of course had the same physical effect. But the 

 power accruing to the experimenter was this. Gun-cotton is 

 easily soluble in ether, the glutinous fluid known as collodion 

 being the result. So, having taken gun-cotton, M. Pasteur 

 for this was the ingenious experimenter used it as a filter; 

 next dissolving the materials of his filter, he liberated into 

 the transparent collodion whatever germs of organic life might 

 be present. Being liberated, he subjected them to micro- 

 scopic examination, and then what he had been in quest of 

 was found. 



Hitherto the group of experiments I have been describing 

 seems far enough away from any practical application. To 



