civilization during the last half century has seen a marked expansion in the extent of the dairy industry.  With this expansion has appeared the necessity for new methods, and dairymen have for years been looking for them.  The last few years have been teaching us that the new methods are to be found along the line of the application of the discoveries of modern bacteriology.  We have been learning that the dairyman is more closely related to bacteria and their activities than almost any other class of persons.  Modern dairying, apart from the matter of keeping the cow, consists largely in trying to prevent bacteria from growing in milk or in stimulating their growth in cream, butter, and cheese.  These chief products of the dairy will be considered separately.

Sources of bacteria in milk.

The first fact that claims our attention is, that milk at the time it is secreted from the udder of the healthy cow contains no bacteria.  Although bacteria are almost ubiquitous, they are not found in the circulating fluids of healthy animals, and are not secreted by their glands.  Milk when first secreted by the milk gland is therefore free from bacteria.  It has taken a long time to demonstrate this fact, but it has been finally satisfactorily proved.  Secondly, it has been demonstrated that practically all of the normal changes which occur in milk after its secretion are caused by the growth of bacteria.  This, too, was long denied, and for quite a number of years after putrefactions and fermentations were generally acknowledged to be caused by the growth of micro-organisms, the changes which occurred in milk were excepted from the rule.  The uniformity with which milk will sour, and the difficulty, or seeming impossibility, of preventing this change, led to the belief that the souring of milk was a normal change characteristic of milk, just as clotting is characteristic of blood.  This was, however, eventually disproved, and it was finally demonstrated that, beyond a few physical changes connected with evaporation and a slight oxidation of the fat, milk, if kept free from bacteria, will undergo no change.  If bacteria are not present, it will remain sweet indefinitely.

But it is impossible to draw milk from the cow in such a manner that it will be free from bacteria except by the use of precautions absolutely impracticable in ordinary dairying.  As milk is commonly drawn, it is sure to be contaminated by bacteria, and by the time it has entered the milk pail it contains frequently as many as half a million, or even a million, bacteria in every cubic inch of the milk.  This seems almost incredible, but it has been demonstrated in many cases and is beyond question.  Since these bacteria are not in the secreted milk, they must come from some external sources, and these sources are the following: