Canning.--Canning is simply a method by which first the bacteria in a substance are killed by heating and then the substance is put into vessels into which no more bacteria may gain entrance. This is usually done at home by boiling the fruit or vegetable to be canned either in salt and water or with sugar and water, either of which substances aids in preventing the growth of bacteria. The time of boiling will be long or short, depending upon the materials to be canned. Some vegetables, as peas, beans, and corn, are very difficult to can, probably because of spores of bacteria which may be attached to them. Fruits, on the other hand, are usually much easier to preserve. After boiling for the proper time, the food, now free from all bacteria, must be put into jars or cans that are themselves absolutely _sterile_ or free from germs. This is done by first boiling the jars, then pouring the boiling hot material into the hot jars and sealing them so as to prevent the entrance of bacteria later. Uses of Canning.--Canning as an industry is of immense importance to mankind. Not only does it provide him with fruits and vegetables at times when he could not otherwise get them, but it also cheapens the cost of such things. It prevents the waste of nature's products at a time when she is most lavish with them, enabling man to store them and utilize them later. Canning has completely changed the life of the sailor and the soldier, who in former times used to suffer from various diseases caused by lack of a proper balance of food. [Illustration: Pasteurizing milk. Why should this be done?] Pasteurization.--Milk is one of the most important food supplies of a great city. It is also one of the most difficult supplies to get in good condition. This is in part due to the fact that milk is produced at long distances from the city and must be brought first from farms to the railroads, then shipped by train, again taken to the milk supply depot by wagon, there bottled, and again shipped by delivery wagons to the consumers. When we remember that much of the milk used in New York City is forty-eight hours old and when we realize that bacteria grow _very_ rapidly in milk, we see the need of finding some way to protect the supply so as to make it safe, particularly for babies and young children. This is done by _pasteurization_, a method named after the French bacteriologist Louis Pasteur. To pasteurize milk we heat it to a temperature of not over 170 deg. Fahrenheit for from ten minutes to half an hour. By such a process all harmful germs will be killed and the keeping qualities of the milk greatly lengthened. Most large milk companies pasteurize their city supply by a rapid pasteurization at a much higher temperature, but this method slightly changes the flavor of the milk.