332 
acidity is as high as 0.23 per cent. Eideal’s observations agree very 
nearly Tvith our own. 
According to Stokes (3) milk which has an acidity somewhat less 
than 0.3 ]Der cent of lactic acid will coagulate on boiling. He records 
the fact, however, that 3 samples of milk containing as much as 
0.54 per cent of lactic acid did not coagulate on boiling. 
Eichmond (2) has been able to confirm Stokes’s results almost 
absolutely. He found fresh milk to have an acidity of 20 degrees, 
corresponding to 0.18 per cent lactic acid. According to him milk 
curdles on boiling when it has an acidity of 33 degrees, corresponding 
to 0.297 jDer cent of lactic acid. 
Eevis and Payne (IT) have shown that at the moment when the 
caseinogen is precipitated the calcium triphosphate has been elim^- 
inated, and that the combination of caseinogen with lactic acid has 
reached a maximum. 
It is evident therefore from our results and those obtained by 
other observers that the coagulation of milk is dependent on several 
factors, among which are : time, temperature, degree of acidity, 
quantity and nature of the calciimi salts, etc.; and that in order 
to avoid accidents resulting from curdling in the pasteurization of 
milk the only safe rule to follow is to determine the effect of heat 
on small samples of the milk which it is proposed to pasteurize, or 
better still, to pasteurize the milk as soon as it is drawn from the cow. 
Another important change in milk effected by heat is the destruc- 
tion of the bacteria and other micro-organisms normally present in 
fresh milk, including of course those pathogenic forms which fre- 
quently gain access to milk and cause the spread of infections through 
this medium or which give rise directly to highly poisonous sub- 
stances. For obvious reasons therefore this phase of the subject, 
namely, the pasteurization of milk, has received a great deal of 
attention during recent years at the hands of dairymen and sanita- 
rians. It is foreign to the immediate scope of this communication, 
however, to enter upon a discussion of this subject. Suffice it to 
say in this connection that pasteurization either checks or hinders 
those changes which occur in milk as the result of the life and growth 
therein of micro-organisms, and affords more or less adequate pro- 
tection against the spread of microbic diseases through the medium 
of milk. According to Pasteur (18) milk can be sterilized by heating 
it to about 110° C. and Duclaux kept milk five years by heating it to 
120° C. and preserving it in vessels which had been exhausted of air. 
Effect of Heat ox Miek Exzymes. 
Another less obHous change in milk which is brought about by 
the action of heat is the destruction of the enzymes normally present 
in fresh milk. Like all enzymes those contained in fresh milk are 
