ure must be incurred, and the business must pay for such
expert service; hence, certified milk must be sold at high prices,
twelve to twenty-five cents per quart. This price makes it a special
product and its use is confined mainly to infant feeding.

The bacterial standard for certified milk is usually 10,000 bacteria
per cubic centimeter. It is only by the exercise of the greatest
care at every point that the bacterial content can be kept below
this maximum.

The term "certified milk" has been registered by Mr. Francisco of
New Jersey, who was the first to engage in the production of such
milk under the direction of the Medical Milk Commission of Essex
County, New Jersey. The use of the term is allowed when the milk is
produced under the regulation of any Medical Milk Commission.

Most certified milk is now produced on fancy dairy farms conducted
by wealthy men. The barns and other equipment are the best that can
be obtained, and the methods employed, as far as cleanliness is
concerned, are extreme. In some of the dairies the bacterial content
is reduced to a few hundred per cubic centimeter, or to that which
is derived from the interior of the udder. Such milk will, when well
refrigerated, keep for long periods of time. It is a not uncommon
thing for such milk to keep perfectly sweet for ten to fifteen days.

=Tests for the quality of milk.= At the milk depot and elsewhere, it
is frequently desired to determine the bacterial condition of the
milk in a less refined manner than by the plate cultures of the
bacteriologist, which require a large amount of time for their
preparation and do not yield any positive information for at least
twenty-four hours. There are a number of such tests that may be
applied.

[Illustration: Fig. 39.--Sediment Testers.

In the use of the apparatus on the right, increased air pressure is
used to hasten the filtering process; the same is accomplished in
the apparatus shown in the center by warming the milk by the
injection of steam between the walls of the double jacket.]

1. _Dirt or sediment test._ This is made by filtering a pint of the
mixed milk through a small disc of absorbent cotton. The insoluble
dirt is retained and imparts a color to the cotton, the shade of
which is dependent on the amount of dirt (P. 45). Since it is
impossible to have dirt without bacteria, it is evident that milks
containing a large amount of dirt will be high in bacteria. The
reverse, however, is not