It is well to remember that there are bacteria in all ordinary milk, and that if the milk is from healthy cows and is kept clean and cold these bacteria are harmless. Most of them are the lactic acid bacteria, which change the milk sugar into acid. When the milk has attained a certain degree of acidity, the lactic acid bacteria are unable to thrive and the souring process is slowed up and finally stopped. Most of the other bacteria in milk perish when lactic acid is formed. This is why stale sweet milk is often harmful, when the same kind of milk allowed to sour can be taken with impunity. If the milk is kept in a cold place the bacteria multiply slowly. If it is kept in a warm place they increase in numbers at a rate that is marvelous, and consequently the milk sours much sooner. Even if the milk is kept cold, bacterial growth will soon take place, but it will perhaps not be lactic acid bacteria. It may be a form that causes the milk to become ropy and slimy or one that gives it a bad odor. Bacteria are like other forms of vegetation, such as grass, weeds, flowers and trees, in that some flourish best under one condition and others under dissimilar conditions, and they struggle one against the other for subsistence and existence. Like flowers there are thousands of different forms of bacteria and they vary according to their food and environment. Peculiar odors in milk generally come from certain kinds of food given to the cows, such as turnips; from bacterial action; or from flavors absorbed from other foods or from odors in the air. Milk should not be exposed to odorous substances, for it becomes tainted very quickly. Sometimes yeast finds its way into milk and causes decomposition of the sugar with the formation of carbon dioxide and alcohol. A count of the bacteria in milk often serves a good purpose, for it shows whether it is good and has had proper care. The consumers have a right to demand milk low in bacteria, for if no preservatives have been used, that means clean milk. If we could live in our pristine state of beatific bliss, if such it was, we would not have to use milk after childhood is past, but our present condition demands the use of easily digested foods and to many milk is almost a necessity. The milk in the udder of a healthy cow is almost surely free from bacteria, but the moment it is exposed to the air these little beings start to drop into the fluid.