TRANSPORTATION PROBLEMS. 171 
The Use of Preservatives. It is easy to add to the milk 
various chemicals which will prevent the growth of bacteria, and 
consequently preserve the milk. Many such substances have 
been used. There are quite a number of preservatives on the 
market which are sold to the farmer to assist him in preserving his 
milk. The basis of most of these is either boracic acid, sali^lic 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 unhesitatingly 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. 
i. Sterilization. This means the use of heat sufficient to de- 
troy 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, sub- 
jected to steam under pressure, and subsequently hermetically 
sealed while still within the closed vessels. Such a procedure 
inevitably makes the milk rather expensive. But milk thus pre- 
pared is supposed to be germ-free, and, consequently, should keep 
indefinitely. Unfortunately, even these temperatures do not al- 
ways destroy all the spores, for some samples of milk thus treated 
have subsequently undergone fermentative changes, due to the 
germination of the spores that are left alive. Further, it has ap- 
peared 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 containing 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, 
