164 THE FLOATING-MATTER OF THE AIR. 



at the time, the absence of agitation which permitted 

 the germs to sink into its tubes was the cause of the 

 quicker contamination of the lower tray. No other 

 cause appeared to me assignable in the present instance. 

 By some means or other germs had insinuated them- 

 selves into my closed chamber, where the tranquillity of 

 the air permitted them to sink into the infusion, and 

 thus produce effects in advance of those produced by 

 the unquiet air outside. So, at all events, I reasoned. 



But how could the germs get into the chamber ? I 

 could, at the moment, fix only upon one way. The 

 weather had changed from warm to cold and from cold 

 to warm. This genial outside temperature sometimes 

 caused the air surrounding the infusions to rise to up- 

 wards of 90° Fahr., and we had often to work in this 

 heat. To moderate it, I sometimes partially turned off 

 the gas, thus lowering the temperature of the room 10° 

 or more. The contraction of the air within the closed 

 chambers followed as a matter of course, and the bent 

 tubes being open, I thought the entrance of the external 

 air might be sufficiently rapid to carry germs along 

 with it. 



A new chamber of six tubes was therefore prepared 

 upstairs, three of its tubes being charged with cucum- 

 ber- and three with turnip-infusion on the 27th of 

 November. The pipette funnel and the bent tubes 

 were plugged above with cotton-wool, which was not 

 removed from them afterwards. I took care, moreover, 

 not to alter the gas-stoves in any way. My care was 

 nugatory. In three days every tube of the six w^s 

 laden with life. Another chamber of six tubes, charged 

 on the 30th of November with cucumber-infusion, and 

 two additional ones prepared on December 1st, shared 

 the same fate. 



Slices of cucumber were next digested for three 



