Page:Popular Science Monthly Volume 40.djvu/789

Rh to it a certainty, and it has only been by exercising the most extraordinary precautions that scientists have in a few cases succeeded in obtaining milk directly from the cow in such a way as to avoid its becoming contaminated during the milking. At the same time much may be accomplished by cleanliness in the barn and the dairy. The presence of disease-germs in the milk is always to be traced to filth or to carelessness in handling the milk, or to diseased cows. Typhoid-fever germs, for instance, can only get into the milk from some unusual source, and tuberculosis germs only from diseased cows. If it were possible to enforce cleanliness in the barns and dairies, and to obtain sufficient care in the handling of milk, we should have no more epidemics spread through the milk-supply. But, in the present state of public ignorance and carelessness as to health, such an enforcement is an impossibility. In our cities and large towns, therefore, the milk supply must be looked upon as one of the fertile sources for the spread of disease, and it behooves every one to look carefully to the condition of the milk he drinks during times of epidemics, especially of those affecting the digestive organs.

But even with the most extreme care it is impossible for our milkmen to avoid the presence of the more common forms of micro-organisms which will sour the milk. Recognizing, then, that bacteria are sure to get into the milk, we may next ask if there is no way of destroying them after they get in. If we can kill these bacteria, we can of course preserve the milk longer and render it more healthful. It is easy enough to kill the bacteria though every method is open to certain objections. Various chemicals have been suggested for poisoning the bacteria, or at least for delaying their growth, but they are one and all to be condemned, as likely to do more harm than good.

A safer and more effective method for preserving milk is by the use of heat. All bacteria may be killed by heat, and then, if the milk be kept in tightly closed bottles, it will keep sweet indefinitely. For this purpose many sterilizing machines have been invented in the last few years, all based upon the same principle, but differing much in detail. In all cases the milk is subjected to a high heat. Most of them simply heat the milk to a boiling temperature by means of steam or boiling water, but a few, by boiling under pressure, contrive to raise the temperature considerably above boiling water. Although many forms of apparatus have been devised for simplifying the matter, no apparatus is really needed for sterilization. All that is necessary is to put the milk into bottles and boil it for ten minutes with the mouth of the bottle open, then close the mouth and steam it for ten minutes more. This method of sterilization will not kill all of the bacteria in the milk, but it will kill all the disease-germs which are likely