Hard fats include human fat, fat of beef and mutton, and wax. Lard and butter are soft fats. Liquid fats include all animal and plant oils that stay in a fluid State at ordinary temperatures. Hard and soft fats may be liquefied by heat, and liquid fats may be hardened by a chemical process called hydrogenation. By this process the liquid fat is kept in a solid form and does not spoil. The most important of the fatty acids are palmitic, stearic, and oleic acids. There are comparatively large amounts of palmitic and stearic acids in hard fats. Liquid fats, such as olive and cottonseed oil, contain more oleic acids. Mineral oils, such as petroleum, are not considered fats because they cannot be absorbed in the body. Most fats may be dissolved in chloroform, ether, gasoline, and benzine. These substances, therefore, are often used to remove grease stains from clothing.

      Fats in the Diet. Fats furnish more than twice as much fuel and energy for the body as the same amount of proteins or carbohydrates. This is because fats contain more carbon and hydrogen. When carbon and hydrogen burn, they give off huge amounts of heat. A pure fat has a fuel value of 4,040 calories a pound, while Sugar, a typical carbohydrate, has a fuel value of 1,820 calories a pound. In cold countries fat is more necessary to the diet than in warm countries. Less fat should be eaten in summer, as the body demands for heat are less then. In winter more fat should be eaten so that the body can resist cold. Eskimos eat a great amount of fat or tallow in order to store up fat in their bodies. This serves as a source of heat and also as an insulation against cold. All fat not used for fuel energy, or growth, is stored in the tissues as body fat.

      Nearly all the fat found in food is digestible. But it takes a long time for digestive juices to work on fat. Thus fried foods that are covered with fat will be digested more slowly than boiled foods. Fatty foods therefore keep a person from feeling hungry for a longer period of time than a diet without fat. Cooking other foods in fat makes the food tasty, but care must be taken not to overheat the fat coverod foods, as this makes them less digestible. Fat is not digested in the stomach, but in the small intestine. Vitamins A and D are found in most fats. Vitamin A is necessary for growth, and Vitamin D prevents the disease called rickets. These vitamins are especially plentiful in halibut, shark, and cod liver oils, and in butter and cream.

     The bodies of some persons do not burn up all the fat that they take in, but store up the excess fat in the body tissues. Such persons generally gain too much weight. In general a person can lose weight either by eating less, or by doing more physical exercise, or both. The principle used in dieting is to regulate the diet so that the body must draw on its reserve stores of fat. Each person must follow the diet best suited to his own physical conditi6n. The advice of a doctor is needed by a person who is greatly overweight or underweight. See Diet.

Industrial Uses. Besides its importance as a food, fat is used as a base for all soap. Coconut oil and palm oil are two of the chief fats used in making soaps. Fat is also a chief ingredient in the making of candles. Fats are used in the making of paints, varnishes, leather, artificial rubber, polishes, waxes, carbon paper, salves, insecticides, lubricants, cosmetics, drugs, dyes, petroleum, and synthetic resin. Another important use for fat is as a cooking oil. Some fats, such as castor oil and cod liver oil, are used as medicines. The glycerin in fat is an important ingredient of explosives. During World War II housewives saved used cooking fats to be used in making soap; and its resulting by-product, glycerin, was used in making explosives.