~Commercial pasteurizers.~ The two methods of pasteurization practiced commercially for the preservation of milk and cream have been developed because of the two types of machinery now in use. Apparatus constructed on the reservoir or tank principle permits of the retention of the milk for any desired period of time. Therefore, a lower temperature can be employed in the treatment. In those machines 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.