764 THE POPULAR SCIENCE MONTHLY. 



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 tumbler- 

 ful, and by the time it has reached the city consumer it will fre- 

 quently contain fifty millions to the quart. Now, if the milk 

 while in the cow contains no bacteria, it follows that this numer- 

 ous crop must have been planted in the milk during the milking 

 or subsequently. At first thought it seems hardly possible to be- 

 lieve 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 open- 

 ing, 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. 



