The Work of Acidity in Ckeese-makiiig. 
277 
of knowledge on this subject which will enable him, by the aid 
of a few still more simple apjiliances, to work out a series of most 
instructive experiments for himself. But, even without such an 
appliance for measuring with precision the amount of acidity- 
present in milk and whey, anyone who will procure a few of the 
blue and red litmus-papers, which may be obtained from any 
pharmaceutical chemist, may by their aid obtain a good deal of 
useful knowledge on this matter. Let us see what can be done 
in this direction. 
If we take a sample of perfectly fresh milk from a healthy 
cow and test it with the litmus-papers referred to above, we shall 
find that whilst the blue paper is feebly reddened to a purple 
tint, the red paper is also blued by it to the same hue. This 
paradoxical effect (technically known as the amphotenc reaction 
of milk) is due to the presence in the milk at the same time of 
certain alkalme salts (phosphates) which act upon the blue 
litmus-paper in the same way as free alkalies do, and also, in 
all probability, to that of a small quantity of free carbonic and 
lactic acids, the result of fermentative action in the milk which 
has commenced even in the udder of the cow. 
The clearness of the alkaline reaction may be increased by 
boiling the milk, but it gradually disappears as the souring of 
the milk proceeds. 
K we are able to measure the exact acidity of the milk by 
means of the appliance referred to above, we shall probably find 
that it is not more than two or three degrees. The alkalinity, 
on the other hand, cannot be directly measured, unless we can 
avail ourselves of other resources of the chemist, and its chief 
importance lies in the proof which it affords that even in per- 
fectly sound milk, at the earliest stage after its separation from 
the udder of the cow, a certain feeble trace of it is to be dis- 
covered by the aid of the reagent which we have employed for 
the purpose. 
If. now, the sample ot milk be left to itself and be examined 
from time to time with both the blue and red litmus-papers, it 
will be found, as indicated above, that the red litmus-paper 
gradually ceases to exhibit any change in tint whilst the blue 
paper becomes redder and redder. By the aid of an acidometer 
the increase of acidity in the milk can be minutely followed 
until it reaches from 15^ to 20^, at about which stage, according 
to the temperature at which it has been kept, spontaneous 
curding of the milk will take place. 
Upon what, it may be asked, does this curding depend ? 
Clearly, in a primary degree, on the increase of acidity. That 
this is the case may be proved by taking another sample of fresh 
