QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 13 



filtered water from Lake Zurich and examined specimens from 

 time to time with the following results : 



Colonies per c.c. 



Immediately after filling the flask . . :'&"'. 143 



After 24 hours . . . . .'..., 12,457 



3 days . . . . . . . 328,543 



8 . . . . . . , * 233,452 



.. 17 . . . . ..' . . 17,436 



,, 70 2,500 



A low temperature appears to arrest the multiplication of 

 micro-organisms. Miquel showed that when a water was kept 

 below 5 C. the number of bacteria did not appreciably alter. 

 The number was kept relatively about the same, not because the 

 organisms were unable to increase but because they were un- 

 equally affected by temperature, the deaths of some species being- 

 balanced by the births of others. 



Wolffhugel and Reidel stated that there was a difference in 

 the multiplication of micro-organisms in water according as the 

 samples were exposed to the air or not. It was generally found 

 that in the vessels closed with india-rubber stoppers somewhat 

 less multiplication took place than in those which were closed 

 with cotton-wool plugs. The apparently more flourishing con- 

 dition of the micro-organisms in the latter case seemed to be 

 due to the interchange of air being less restricted. 



Frankland obtained the same results when working with water 

 from the Thames and Loch Katrine. 



The diminution in the number of bacteria in a sample of water 

 kept for many days, appears to be due partly to some one or 

 other of the food materials necessary for the growth of certain 

 varieties being used up, and partly to the products of the 

 bacteria being harmful so that the water becomes both 

 " impoverished and poisoned.' 1 



Sirotinin made some experiments on the impoverishment of 

 media. He planted out the Bacillus typhosus on a thin layer of 

 gelatine and obtained a good growth ; on removing the first 

 crop, however, he was unable to obtain a second until the gelatine 

 already used had been melted and mixed with 1 per cent, 

 peptone and O'l per cent, meat extract. Similar results were 



