definite way of telling whether or not one of these has been used,
except by a chemical analysis. However, if milk does not sour within a
reasonable time when no precautions have been taken to keep it sweet, it
should be looked on with suspicion, for it undoubtedly contains a
preservative.

25. FRESHNESS OF MILK.--To be most satisfactory for all purposes, milk
should be absolutely fresh. However, it is almost impossible to obtain
milk in this condition, because it is generally sold at a distance from
the source of supply. Milk that is sold in small towns and cities is
usually 12 and often 18 to 21 hours old when it is delivered; whereas,
in large cities, where the demand is so great that milk must be shipped
from great distances, it is often 24 to 36 or even 48 hours old when it
reaches the consumer. In order that milk may remain sweet long enough to
permit it to be delivered at places so far removed from the source of
supply, it must be handled and cared for in the cleanest possible way by
the dealers. Likewise, if the housewife desires to get the best results
from it, she must follow the same plan, cooling it immediately on
delivery and keeping it cool until it is consumed. The freshness of milk
can be determined only by the length of time it will remain sweet when
proper care is given to it.

26. CLEANLINESS OF MILK.--Milk may be of the right composition, free
from all adulteration, and as fresh as it is possible to obtain it, but
unless it is clean, it is an injurious food. Milk is rendered unclean or
impure by dirt. In reality, there are two kinds of dirt that may be
present in milk, and it is important to know just what these are and
what effect they have on milk.

27. The less harmful of the two kinds of dirt is the visible dirt that