that it has until recently been considered a normal character of milk. The last twenty years have, however, demonstrated for us that it is universally caused by bacteria growth. The souring of milk is simply the formation in it of a certain amount of lactic acid, and the acid precipitates the casein of milk just as any other acid would do, and thus forms the curd. But it is bacteria which produce the lactic acid. The presence of micro-organisms in milk was first noticed fifty years ago by Fuchs, but it was not till twenty years later that Pasteur succeeded in showing that these organisms could really produce lactic acid and thus might be the cause of the souring of milk. Fifteen years more were required to show that they were the sole cause of the souring of milk, and to demonstrate the further important point that milk when drawn from the healthy cow contains no bacteria and has therefore no tendency to sour or undergo other unpleasant changes. Since this was first shown by Lister, in 1873, numerous observers have so successfully verified the conclusions of Pasteur and Lister that no possibility of doubt longer remains, and we now know that under normal conditions the milk while in the mammary gland of the healthy cow is free from bacteria, and we have abundant proof that such milk will never sour nor ferment if kept free from bacteria contamination.
Absolutely pure milk is, then, free from bacteria; but when we examine milk that has been standing for a few hours the number of bacteria found in it is almost incredible. By the time that it is five or six hours old milk will contain millions for each tumblerful, and by the time it has reached the city consumer it will frequently contain fifty millions to the quart. Now, if the milk while in the cow contains no bacteria, it follows that this numerous crop must have been planted in the milk during the milking or subsequently. At first thought it seems hardly possible to believe that this immense number of bacteria could have found their way into the milk since the milking. But when we learn that they are abundant in the air; that they are crowded in every particle of dust clinging to the hairs of the cow; that they are always present in the milk-duct for a short distance from its opening, living there in the remains of the milk left from the last milking; that the milk-pail in which the milk is drawn can not be washed clear of them by any ordinary methods; that the milk-cans will always contain them in cracks and chinks even after the most thorough cleansing; that they are always on the hands of the milker; and when, in addition to all this, we learn that bacteria multiply so fast that by actual experiment a single individual may in the course of six hours give rise to three thousand progeny—it no longer remains a marvel that their number is so great in milk of a few hours' standing.
