The work of the germ in the dairy is not, however, confined to souring the milk. Certain kinds of germs give to the different sorts of cheeses their marked flavors and to butter its flavor. If the right germ is present, cheese or butter gets a proper flavor. Sometimes undesirable germs gain entrance and give flavors that we do not like. Such germs produce cheese or butter diseases. "Bitter butter" is one of these diseases. To keep out all unpleasant meddlers, thoroughly cleanse and scald every utensil. EXERCISE What causes milk to sour? Why do unclean utensils affect the milk? How should milk be cared for to prevent its souring? Prepare two samples, one carefully, the other carelessly. Place them side by side. Which keeps longer? Why? SECTION LXIII. THE BABCOCK MILK-TESTER It is not sufficient for a farmer or a dairyman to know how much milk each of his cows yields. He should also know how rich the milk is in butter-fat. Wide-awake makers of butter and cheese now buy milk, not by the pound or by the gallon, but by the amount of butter-fat contained in each pound or gallon of milk. A gallon of milk containing four and a half per cent of fat will consequently be worth more than a gallon containing only three per cent of fat. So it may happen that a cow giving only two gallons of milk may pay a butter-maker more than a cow giving three gallons of milk. Of course it is easy to weigh or measure the quantity of milk given by a cow, and most milkers keep this record; but until recent years there was no way to find out the amount of fat in a cow's milk except by a slow and costly chemical test. Dairymen could only guess at the richness of milk. In 1890 Dr. S. M. Babcock of the Wisconsin Experiment Station invented a wonderful little machine that quickly and cheaply measures the fat in milk. Few machines are more useful. So desirous was Dr. Babcock of helping the farmers that he would not add to the cost of his machine by taking out a patent on his invention. His only reward has been the fame won by the invention of the machine, which bears his name. This most useful tester is now made in various sizes so that every handler of milk may buy one suited to his needs and do his own testing at very little cost. The operation of the machine is very simple. Suppose that the members of the class studying this book have been asked to take a Babcock machine and test the milk of a small herd of cows. They can readily do so by following these directions: