eria cannot grow,
but other types, such as certain of the putrefactive forms grow
slowly; the milk may, therefore, have no objectionable odor or taste
and yet be swarming with bacteria. In cities the practice is
followed of placing cream in cold-storage during the cooler periods
of summer in preparation for an increased demand, during hot weather
or on holidays. It seems probable that poisoning from ice cream may,
at times, be due to the use of such cream.

=Preservation by the use of antiseptics.= Many chemical substances
prevent the growth of bacteria when added to food supplies; such
substances thus used are called _preservatives_. In the past some of
these have been used in milk to a great extent, but at present, on
account of stringent pure food laws, they are employed only to a
slight extent. There is a great temptation for the small milk dealer
in the city to employ them to preserve the excess of milk from day
to day, as through the use of a few cents worth of some preparation,
many dollars worth of milk may be kept from spoiling until it can be
sold to the unsuspecting consumer.

Formalin has been most widely used in milk because it is a most
efficient preservative; it is cheap and cannot be detected by the
consumer, although it injures the digestibility of the casein. One
ounce will keep one thousand pounds of milk sweet for twenty-four to
forty-eight hours. Borax, boric acid, and salicylic acid have also
been used, but these substances must be employed in much larger
quantities than formalin. Bicarbonate of soda has sometimes been
used although it is not a true preservative. Its effect is based
upon the neutralization of the acid produced by bacterial growth.
The treated milk does not taste sour so quickly, and the curdling of
the milk is also delayed.

Many proprietary compounds for milk preservation have been placed on
the market in the past, but the use of all of these is illegal in
most states. The federal law also prohibits their use in all dairy
products that pass into interstate commerce.

Within recent years a method for the preservation of milk was
introduced by a Danish engineer, Budde, which consists of adding to
milk a very small amount of peroxid of hydrogen which is a very
efficient antiseptic. The peroxid is decomposed by some substance in
the milk; the products of decomposition being water and free oxygen.
The peroxid together with the application of heat at a comparatively
low temperatur