EXPERIMENT STATION BULLETINS. 



297 



usually after the acidity begins to increase, this increase is much more rapid than in 

 the case of the glass Fernbach flask. These tables also show that usually the milk 

 exposed to the air under the conditions given sours more rapidly than when confined; 

 occasionally, but seldom, as in Test III, the reverse is the case. I have inserted this 

 test as one that is somewhat rare. Tests I and II represent the usual results obtained. 

 In connection with the acidity of milk, several problems present themselves which cast 

 some light upon its gas-content. Unagrated milk containing 60 per cent carbon dioxide 

 does not act the same as milk which has had GO per cent carbon dioxide maintained 

 over its surface and with which it was mixed. We find that milk after milking has 

 about GO per cent carbon dioxide which we estimate as free carbon dioxide and this 

 will give a low estimate of acidity when phenolphthalein is used as an indicator, in 

 fact the acidity is regarded as normal, but as soon as the free carbon dioxide is passed 

 into the milk or the same percentage maintained over it immediately the acidity 

 ascends rapidly. This condition may be explained by the solubility of carbon dioxide. 

 Normal milk cannot be considered saturated with this gas. By passing carbon dioxide 

 into the milk the acidity to phenolphthalein rises from 13° (.117 per cent) acid to 19° 

 (.171 per cent) in ten minutes, to 28° (.252 per cent) in thirty minutes, to 38° (.342 

 per cent) in seventy minutes. This does not correspond in any manner to confined 

 milk other than the fact that confined milk does not fall in acidity until fermenta- 

 tion sets in. 



XXVIII. DO UNAERATED CONDITIONS OF MILK TEND TO HOLD BACTERIA IN CHECK? 



In these tests, three of which are recorded as representative, I have endeavored to 

 study the germ content in milk subjected to exactly the same conditions as we found 

 in the previous study of acidity. The same sample of milk was used in each instance 

 and great care exercised in securing exact dilutions and measures. In each count 

 several plates were made from each sample and the average taken. 



TABLE XXIV. 



DO UNAERATED CONDITIONS OF MILK TEND TO HOLD BACTERIA IN CHECK? 



TEST I. 



After milking 

 After 21 hours 

 After 39 hours 

 After 48 hours 

 After 87 hours 



Confined. 



8,400 



31,950 



756.000 



1,096,000 



1,747,500,000 



Exposed 

 over glass. 



8,400 



336,300 



84,292,500 



215,205,000 

 Curdled. 

 5,142,500,000 



Exposed 

 over tin. 



8,400 



14,096,400 



125,650,500 

 < urdled. 

 250,739,000 



10,037,500,000 



TEST II. 



After milking 



After 20 J hours. .. 



After 24 hours 



After 40 hours 



After 48 hours 



After 64 hours — 



After 69 hours 



After 88 hours 



After 96 hours 



38 



Confined. 



6.620 



31,850 



35.950 



360,000 



773,000 



160,000,000 



155,000,000 



112,500,000 



75,000,000 



Exposed 



Exposed 



over glass. over tin. 



6,620 



46,750 



62,250 



53,282,000 



109,999,000 

 Curdled. 

 877,500,000 



2,570,000,000 



1,760,000,000 



1,517,500,000 



6,620 



331,400 



798,150 



914,895,000 

 Curdled. 

 1,069,205,000 



3,722,500,000 



1,702,500,000 



1,650,000,000 



1,227,500,000 



