If we include milk, cheese, and eggs in the vegetarian diet, the objections to it partially disappear; and it would be well if it were much more widely known, especially among the poor, that on these, together with vegetables, health can be maintained with the addition of little or no meat. =The Determination of Diet.=—The first principle in making a dietary is that it =must be mixed=, containing all the necessary constituents, proteids, hydrocarbons, carbohydrates, water, and salts. No one of these alone will support life for any considerable period. Carbohydrates (sugar and starch) can be most easily dispensed with; fats, on the other hand, are essential for the maintenance of health. The next point is to ascertain =the proportion= in which these different foods are required. _Salts_ are commonly taken with other foods, common salt being the only one taken alone. The amount required is given on p. 7. The amount of _water_ required varies with the season of the year, the amount of exercise and perspiration, and other factors. As a rule, not more than two pints of water are required per day, and still less if fruit is freely taken. We may therefore confine our attention to the carbonaceous and nitrogenous foods, and try to ascertain the amount of each of these required. Every diet must be subjected to the following tests, to fully ascertain its value:— 1. =The Chemical Test.=—The metabolism undergone by food in the body being essentially a process of oxidation (though partially modified and incomplete), the amount of heat yielded on complete combustion of a food may be taken as a measure of its value as a source of energy, of which heat and work are convertible forms. The standard of heat production is the =calorie=, the amount of heat required to raise the temperature of one gramme of water 1° C. This is the small calorie. The kilo-calorie (called the =Calorie=) is the amount of heat required to raise 1 kilo (1 litre) of water 1° C., or 1 lb. of water 4° F. In calculations on this basis, allowance must be made for foods which are incompletely oxidised in the body. Rubner has shown that the heat value of 1 gramme (=15½ grains) of each of the chief food stuffs is as follows:— Proteid 4.1 Calories. Carbohydrates 4.1 „ Fat 9.3 „ The method of applying this standard to a food is as follows: the percentage of proteid or carbohydrate given in the following table is multiplied by 4.1, and the percentage of fat by 9.3:—