Dairying is one of the most primitive of our industries. From the very earliest period, ever since man began to keep domestic cattle, he has been familiar with dairying. During these many centuries certain methods of procedure have been developed which produce desired results. These methods, however, have been devised simply from the accumulation of experience, with very little knowledge as to the reasons underlying them. The methods of past centuries are, however, ceasing to be satisfactory. The advance of our 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.