144 THE FLOATING-MATTER OF THE AIR. 



§ 4. Hay -infusions. Experiments ivith Cohn's 

 Tubes. 



For reasons given by himself,' Professor Cohn de- 

 viated from the method of experiment pursued by Dr. 

 Eoberts, employing, instead of the pipette- 

 FiG. 15. bulbs, flasks, the nature of which will be under- 

 stood from the following description. Let a 

 zone of a common test-tube, about one-third 

 of its length from its open end, be softened 

 by heat, and let the softened glass be drawn 

 out so as to form a tube of much narrower 

 bore than the original test-tube. Thus modi- 

 fied, the tube would consist of an elongated 

 bulb below and an open funnel above, both 

 being connected by a narrow neck (see fig. 15). 

 Professor Cohn filled the elongated bulb to 

 about two-thirds of its volume with hay- 

 infusion, plunged his bulbs in water, raised the 

 water to ebullition, and continued the boiling 

 for the required time. The tubes were then 

 removed from their bath, and after being held 

 open for a minute or two so as to allow the 

 water condensed in their necks to evaporate, 

 the funnel was plugged with cotton-wool. 



Professor Cohn considers that all possibility 

 of external contamination is here shut out.^ By 

 his method, therefore, I wished to check the results above 



' Beitrllge, July 187G, p. 25G. 



2 ' Elie ich iUjcr die Organiamen berichte, wclche sich in den 

 gekochten Aufgiissen entwickelten, will ich bemerken, dass an eine 

 naclitragliclie Infection derselben durch von aussen nach dem 

 Kochcn eingeschlcppte Keime bei unscren Versuchcn nicht zu 

 donken ist' (p. 259). I may remark that, with an atmosphere like 

 that in which my recent ex[)crimeiits were conducted, (here would 

 be no chance of escape for an infusion thus handled. 



