The organism that is invested with most interest in this connection is the tubercle bacillus. On account of its more or less frequent occurrence in milk and its reputed high powers of resistance, it may well be taken as a standard in pasteurizing. ~Thermal death limits of tubercle bacillus.~ Concerning the exact temperature at which this germ is destroyed there is considerable difference of opinion. Part of this arises from the inherent difficulty in determining exactly when the organism is killed (due to its failure to grow readily on artificial media), and part from the lack of uniform conditions of exposure. The standards that previously have been most generally accepted are those of De Man,[137] who found that thirty minutes exposure at 149 deg. F., fifteen minutes at 155 deg. F., or ten minutes at 167 deg. F., sufficed to destroy this germ. More recently it has been demonstrated,[138] and these results confirmed,[139] that if tuberculous milk is heated in closed receptacles where the surface pellicle does not form, the vitality of this disease germ is destroyed at 140 deg. F. in 10-15 minutes, while an exposure at 160 deg. F. requires only about one minute.[140] If the conditions of heating are such that the surface of the milk is exposed to the air, the resistance of bacteria is greatly increased. When heated in open vessels Smith found that the tubercle organism was not killed in some cases where the exposure was made for at least an hour. Russell and Hastings[141] have shown an instance where the thermal death-point of a micrococcus isolated from pasteurized milk was increased 12.5 deg. F., by heating it under conditions that permitted of the formation of the scalded layer. It is therefore apparent that apparatus used for pasteurization should be constructed so as to avoid this defect. ~Methods of treatment.~ Two different systems of pasteurization have grown up in the treatment of milk. One of these has been developed from the hygienic or sanitary aspect of the problem and is used more particularly in the treatment of cream and relatively small milk supplies. The other system has been developed primarily from the commercial point of view where a large amount of milk must be treated in the minimum time. In the first method the milk is heated for a longer period of time, about fifteen minutes at a relatively low temperature from 140 deg.-155 deg. F.; in the other, the milk is exposed to the source of heat only while it is passing rapidly through the apparatus. Naturally, the exposure under such conditions must be made at a considerably higher temperature, usually in the neighborhood of 160 deg. F. The types of apparatus used in these respective processes naturally varies. Where the heating is prolonged, the apparatus employed is built on the principle of a _tank_ or _reservoir_ in which a given volume of milk may be held at any given temperature for any given period of time.