VITALITY OF PUTBEFACTIVE ORGANISMS. 171 



more than an inch from its base. These bags finally 

 broke and scattered their organisms in the subjacent 

 liquid. Thus the conflict went on. 



§ 13. Further 'precautions against Infection. 



At the beginning of December, my attention being 

 keenly aroused by those successive failures, I watched 

 more closely than I had previously done the fiUing of 

 the test-tubes through the pipette. Now and then I 

 noticed minute bubbles of air carried down with the 

 descending infusion. On escaping from the end of the 

 pipette, these small bubbles, I concluded, would break, 

 and scatter such germs as they contained in the air of 

 the chamber. Last year I should have found it difficult 

 to believe that a cause so small could lie at the root of 

 the observed anomalies ; but this year I had learned to 

 respect small causes, and accordingly took measures to 

 effectually exclude the air. 



On December 4th three chambers, which had been 

 previously left qmet for several days, were charged with 

 carefully prepared cucumber-infusion, and two other 

 chambers with turnip-infusion prepared with equal 

 care. The following precautions were taken : — The 

 funnel of the pipette formerly employed was broken 

 off from its shank, and for it was substituted a ' separa- 

 tion-funnel ' with a glass stopcock. This was connected 

 by closely fitting india-rubber tubing with the shank of 

 the pipette. But before the connexion was made, the 

 funnel was filled with the infusion, and the stopcock 

 turned on for a moment, until the liquid issued from 

 the orifice below. The stopcock being then turned off, 

 the flow of the liquid ceased, and the column in the 

 shank below the stopcock was supported by atmospheric 

 pressure. A pinchcock nipped the india-rubber tube at 



