To turn now to the organic constituents. The most important of these from the point of view of quantity is starch, in fact this is the most abundant constituent of ordinary bread. Nor is it in bread only that starch is abundant. It occurs to the extent of from 50 to 70 per cent. in all the cereals, grains, wheat, barley, oats, maize, and rice. Potatoes too contain about 20 per cent. of starch, in fact it is present in most plants. Starch is a white substance which does not dissolve in cold water, but when boiled in water swells up and makes, a paste, which becomes thick and semisolid on cooling. It is this property which makes starch valuable in the laundry. Starch is composed of the chemical elements carbon, hydrogen, and oxygen. When heated in the air it will burn and give out heat, but it does not do so as readily as does fat or oil. It is this property of burning and giving out heat which makes starch valuable as a foodstuff. When eaten in the form of bread, or other article of food, it is first transformed by the digestive juices of the mouth and intestine into sugar, which is then absorbed from the intestine into the blood, and thus distributed to the working parts of the body. Here it is oxidized, not with the visible flame which is usually associated with burning, but gradually and slowly, and with the formation of heat. Some of this heat is required to keep up the temperature of the body. The rest is available for providing the energy necessary to carry on the movements required to keep the body alive and in health. Practically speaking therefore starch in the diet plays the same part as fuel in the steam engine. The food value of starch can in fact be measured in terms of the quantity of heat which a known weight of it can give out on burning. This is done by burning a small pellet of starch in a bomb of compressed oxygen immersed in a measured volume of water. By means of a delicate thermometer the rise of temperature of the water is measured, and it is thus found that one kilogram of starch on burning gives out enough heat to warm 4·1 kilograms of water through one degree. The quantity of heat which warms one kilogram of water through one degree is called one unit of heat or calorie, and the amount of heat given out by burning one kilogram of any substance is called its heat of combustion or fuel-value. Thus the heat of combustion or fuel-value of starch is 4·1 calories.