has been produced under the influence of different species of bacteria. The souring of milk is therefore not a simple or a uniform phenomenon. While it is always the effect of bacteria growth, we recognize many varieties of souring corresponding to the variety of bacteria most abundant in the milk before souring. All this makes little difference to the consumer; in any case the milk is ruined for his purposes, and he is more concerned in preventing such troubles completely than in learning their variety. A remedy seems simple enough. When we have once learned that the whole trouble is caused by bacteria, we see that it is only necessary to keep these organisms out in order to preserve the milk pure and sweet.
From the standpoint of public health also the desirability of freeing milk from these organisms is becoming every day more apparent. It is true that the vast majority of the bacteria in milk are perfectly harmless to the healthy person, even when swallowed in such numbers as above indicated. But, at the same time, it not infrequently happens that disease-germs get into the milk and, finding there a suitable medium for growth, multiply rapidly. They are then served out to all the patrons supplied with the milk. Typhoid fever is certainly disseminated by means of the milk-supply, and there is a growing conviction that the fatal tuberculosis owes much of its prevalence to milk from diseased cows. Other epidemics have also been traced to the same source.
Even if no definite disease-germ chances to be present in the milk, the vast number of harmless forms may render the milk dangerous to all having weak or sensitive digestive organs; for they produce considerable lactic acid, and every one knows that acid is injurious in the food of infants and invalids. The presence of lactic acid is probably a less serious matter than the presence of certain decomposition products which are formed by the same bacteria. These are directly poisonous, and, although they are present in such small quantities that they have no effect on the healthy person, they may be injurious to one whose digestive organs are in a sensitive condition. For a long time doctors have recognized that boiled milk is a safer food for invalids than raw milk, supposing, however, the explanation to be that the cooking renders it more easily digested, just as it does other foods. Recent experiments have shown us that this is not true. On the contrary, boiled milk is less easily digested and absorbed by the system than raw milk. The real reason for the greater safety in drinking boiled milk lies in the fact that it is thus deprived of the disturbing action of the millions of bacteria ordinarily present.
To keep bacteria out of milk is a practical impossibility. Their presence in such quantities in all places renders their access
