iderable height. These are widely used in this country for the
pasteurization of milk and cream for butter making.

Milk that has been heated must be cooled at once by the use of cold
water and ice. In order to economize in the use of both steam and
cooling agents, the so-called regenerative machines were devised.
The essential feature of these machines lies in the fact that the
cold milk inlet and the hot milk outlet are on opposite sides of a
single partition; thus the inflowing cold milk is partially heated
by means of the already treated hot milk which it is desired to
cool.

In order to avoid the disadvantages of the continuous machines,
viz., lack of control, an apparatus has recently been devised which
can handle large quantities of milk, heating the same to any
temperature for any desired time. In such a machine the milk is
first heated in a continuous heater, and is then passed into large
tanks in which it is allowed to remain for the desired time, and
from which it flows over the coolers. Such an apparatus is called a
"holding" machine, and is probably the most feasible type of
pasteurizer now on the market, when all factors are considered. In
some of the continuous machines, an attempt is made to accomplish
the same result, by building the machine so that the milk requires
fifteen to twenty minutes for passage through the machine, but in
all such cases the same disadvantage of variation in rate of flow,
as in other continuous flow type of machines obtains.

=Tests of pasteurizing machines.= It is possible for the operator to
test the rate of flow in a machine, so as to determine whether all
of the milk is heated for a uniform time. This is done most easily
in the following manner: The machine is first filled with water,
heating the same to the desired temperature, and regulating the rate
of flow as it would be if milk was used. The flow of water is then
turned off, and a stream of milk containing a known per cent of fat
admitted to the machine. The time elapsing between the admission of
milk to the machine, and that at which the first sign of turbidity
is noted at the outlet, will be the minimum period necessary for any
portion of the milk to flow through the machine. At frequent
intervals thereafter, samples of the outflowing liquid may be
collected, noting the time at which each sample is taken. The
percentage of fat in the various samples is determined by the
Babcock test; at the moment when all of the