THE MILK SUPPLY OF CITIES. 637 



Under these conditions the bacteria do not commonly have an op- 

 portunity of growing sufficiently to produce their poisonous prod- 

 ucts before the milk is consumed. Practically, then, these bacteria 

 that resist the moderate heat of Pasteurization are of no serious 

 importance in connection with the healthfulness of milk. Pas- 

 teurized milk has been deprived of all its strictly pathogenic bac- 

 teria, and the germs still left will commonly have no opportunity 

 to grow very much before the milk is consumed. It is therefore 

 the confident belief of many that Pasteurization is actually a safer 

 method of treating milk than sterilization. Moreover, the results 

 appear to be equally favorable, for Pasteurization is claimed to pro- 

 duce an effect upon diarrhoeal diseases equal to that of sterilization. 

 But the most important argument for Pasteurization seems to 

 be that it is really practical, and can be introduced upon a scale 

 vastly more extended than can sterilized milk. The practice of 

 Pasteurizing milk has doubtless been followed not a little by pri- 

 vate families, but from the very outset it has appeared that the 

 proper method of dealing with the matter is to treat the milk at a 

 general distributing center, rather than to depend upon the con- 

 sumer to do it. ISTot a few devices have been suggested for accom- 

 plishing the purpose satisfactorily and rapidly. The machines in- 

 vented are planned upon two different principles. In one plan 

 the milk is placed in some large vessel holding many gallons and 

 is here heated, commonly by steam coils. It is allowed to remain 

 here at the desired temperature for twenty minutes to half an hour, 

 and is then cooled. This method is necessarily slow — so slow, in- 

 deed, that it is impractical for use where large amounts of milk 

 must be treated rapidly for general distribution. It probably 

 could not be used for the milk supply of a city. The other method 

 is called that of continuous flow. Here the milk is allowed to flow 

 continuously over a heated surface, which brings it quickly to the 

 desired temperature. It is kept hot for only a short time, how- 

 ever, and it then flows over a cooled surface, where the tempera- 

 ture is brought down again and the milk is finally delivered from 

 the machine in a continuous stream of cooled milk. Great ob- 

 jections have been urged against this process, from the fact that 

 it is not thorough. The milk is retained at the high tempera- 

 ture for such a short time that many of the bacteria are not 

 killed. The Pasteurization is decidedly less thorough than by the 

 other method. But here, again, before condemning the process it 

 is necessary to consider its purposfe. If it is to destroy all the bac- 

 teria, or as large a number of them as is possible, it is of course un- 

 satisfactory. If, however, the purpose is to treat the milk cheaply 

 and rapidly in such a manner as to remove the danger of disease 



