ines where the milk flows
through the heater in a more or less continuous stream, the period of
exposure is necessarily curtailed, thereby necessitating a higher
temperature.

~Reservoir pasteurizers.~ The simplest type of apparatus suitable for
pasteurizing on this principle is where the milk is placed in shotgun
cans and immersed in water heated by steam. Ordinary tanks surrounded
with water spaces can also be used successfully. The Boyd cream ripening
vat has also been tried. In this the milk is heated by a swinging coil
immersed in the vat through which hot water circulates.

In 1894 the writer[142] constructed a tank pasteurizer which consisted
of a long, narrow vat surrounded by a steam-heated water chamber. Both
the milk and the water chambers were provided with mechanical agitators
having a to-and-fro movement.

[Illustration: FIG. 25. Pott's pasteurizer.]

Another machine which has been quite generally introduced is the Potts'
rotating pasteurizer. This apparatus has a central milk chamber that is
surrounded with an outer shell containing hot water. The whole machine
revolves on a horizontal axis, and the cream or milk is thus thoroughly
agitated during the heating process.

~Continuous-flow pasteurizers.~ The demand for greater capacity than can
be secured in the reservoir machines has led to the perfection of
several kinds of apparatus where the milk is heated momentarily as it
flows through the apparatus. Most of these were primarily introduced for
the treatment of cream for butter-making purposes, but they are
frequently employed for the treatment of milk on a large scale in city
milk trade. Many of them are of European origin although of late years
several have been devised in this country.

The general principle of construction is much the same in most of them.
The milk is spread out in a thin sheet, and is treated by passing it
over a surface, heated either with steam directly or preferably with hot
water.

Where steam is used directly, it is impossible to prevent the "scalding
on" of the milk proteids to the heated surface.

In some of these machines (Thiel, Kuehne, Lawrence, De Laval, and
Hochmuth), a ribbed surface is employed over which the milk flows, while
the opposite surface is heated with hot water or steam. Monrad, Lefeldt
and Lentsch employ a centrifugal apparatus in which a thin layer of milk
is heated in a revolving drum.

In some types of apparatus, as in the Miller machine, an A