In this case again we have eleven molecules of water carrying twelve atoms of carbon. This is the chemical composition of starch and sugar food. 2. Fats are also composed of three elements only--carbon, hydrogen, and oxygen. Take the fat of mutton or pork: Carbon. Hydrogen. Oxygen. 21 40 1 All other animal oils and fats are composed of these three elements only. 3. Albuminous substances--meats, beef, mutton, veal, pork, birds, and fish, of all descriptions. 4. Besides these, mineral salts, already mentioned. 5. And lastly, water--of which by far the greatest quantity is consumed. The quantity of food ought to be in amount sufficient to replace the waste products of the body. An amount should be taken into the system equal in kind and quantity to the material expended. Since we know the amount of carbon, hydrogen, nitrogen, oxygen, and the salts that are excreted by the kidneys, skin, and lungs, we may easily calculate the amount of various kinds of food needed to replace them. The outcome being known, the income can be regulated accordingly. The expenditure or waste, we have seen, in daily loss amounts in carbon to about 4,500 grains, and in nitrogen to 300 grains; besides a certain quantity of water, etc. We therefore require starchy substances, meat and fat, water, etc., to replace the quantity lost. Bread, for example, contains 30 per cent of carbon and 1 per cent of nitrogen. If bread alone, therefore, were taken as food, a man would require in order to obtain the requisite nitrogen 30,000 grains, containing of carbon, 9,000 grains; of nitrogen, 300 grains--an excess of carbon above the amount required of 4,500 grains. But a combination of bread and meat would supply much more economically what was necessary: Carbon. Nitrogen. 15,000 grains of bread (rather more than 2 pounds) contains 4,500 grs. 150 grs. 5,000 grains of meat (about 3/4 pounds) contains 500 150 ----- --- 5,000 300 So that 3/4 pounds meat and 2 pounds of bread, or its equivalent, would supply the needful carbon and nitrogen with but little waste. From all these facts it will be plain that a mixed diet is the best and most economical for man; and the result of experience entirely coincides with what might have been anticipated on theoretical grounds only. The quality and quantity of foods to be taken depends largely upon their digestibility. The quantity of food necessary for a healthy man taking free exercise in the open air is as follows: