184 CONTROL OF THE MILK SUPPLY 
market which are sold to the farmer to assist him in preserving his 
milk. The basis of most of these is either boracic acid, salicylic 
acid, or formalin. All of these substances are injurious to man, 
and their use should not be allowed in preserving an article so 
freely used as milk. Such methods are illegal, and are unhesita- 
tingly to be condemned. 
The Use of Heat.—A more legitimate method of obtaining the 
same result is by the use of heat. All bacteria are destroyed by 
heat and therefore, by this simple means, it is possible to kill 
the living organisms in milk, and thus preserve the milk from their 
subsequent action. This has given rise to two chief methods of 
treating milk—sterilization and pasteurization. 
t. Sterilization.— This means the use of heat sufficient to de- 
stroy all bacteria at once. It is perfectly possible to do this, but 
since milk always contains spore-bearing bacteria, sterilization 
requires a high temperature for the purpose. A temperature of 
boiling will not destroy the spores, so it is necessary to heat the 
milk to several degrees above boiling. This involves the use 
of special apparatus, in which bottles of milk can be inclosed in 
special vessels, subjected to steam under pressure, and subse- 
quently hermetically sealed while still within the closed vessels. 
Such a procedure inevitably makes the milk rather expensive. 
Bui milk thus prepared is supposed to be germ-free, and, conse- 
quently, should keep indefinitely. Unfortunately, even these 
temperatures do not always destroy all the spores, for some 
samples of milk thus treated have subsequently undergone fer- 
mentative changes, due to the germination of the spores that are 
left alive. Further, it has appeared that these later changes, due 
to the resisting spores, are frequently such as do not change the 
appearance of the milk to the eye, so that such milk, though con- 
taining bacteria in quantity, will be drunken as pure milk. The 
fermentation has, moreover, filled the milk with bacterial products 
of more or less injurious nature, and consequently the drinking 
of such milk is far worse than drinking fresh milk which is, most 
