THE BEGINNINGS OF LIFE. 273 



at the time. In air obtained from the Rue d'Ulm, in 

 Paris, M. Pasteur encountered, in addition to the parti- 

 cles which he supposed to be of an organized nature, a 

 large quantity and a great variety of foreign ingredients. 

 In these researches M. Pasteur made use, as M. Gigot 

 had done, of an aspirator, by which air was drawn at a 

 definite rate (by a process of filtration) through a glass 

 tube containing a pledget of gun-cotton sufficiently 

 large to arrest all solid particles amongst its fibres. 

 After a known quantity of air had been filtered in 

 this way, the gun-cotton was removed and dissolved in 

 a mixture of aether and alcohol. This was allowed to 

 stand, so that all the solid particles which had been 

 entangled amongst its rneshes, having been liberated, 

 might gradually sink to the bottom of a conical glass. 

 They were afterwards washed several times with dis- 

 tilled water ; and an interval of twelve hours was left 

 between each washing, so that all the corpuscles might 

 have time to subside before the supernatant fluid was 

 withdrawn. After five or six washings the residuum 

 was poured out into a watch-glass, whence any excess 

 of fluid soon evaporated. The particles thus obtained 

 could easily be placed upon an ordinary slide, and ex- 

 amined under the microscope with the aid of various 

 reagents. These examinations convinced M. Pasteur 

 that ordinary air contains a considerable though still a 

 variable number of corpuscles, whose form and struc- 

 ture made him think they were organized. The cor- 

 puscles varied from the smallest appreciable size up to 



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