Diet.--The composition of different articles of food varies. A turnip is not the same as a piece of cheese. It is more watery, and has more fibre in it, and we speak of it as less nutritious. There are, however, in almost all foods certain chemical substances present which have different duties to perform in the body, and which are present in widely different proportions in the various articles we use for food. _Water_ is the most common of these substances. Soups, vegetables, fruits, puddings, are largely water. Some foods contain less of it than others, but on the whole a very large, if not the largest, part of all food consists of water. This large amount is needed. Water makes up two-thirds of the body, and nearly two quarts are given off daily in the various excretions and secretions. If enough be not taken the tissues get dry, and Nature indicates her want in thirst. Another of these substances is _starch_, or its equivalent, sugar. Rice, bread, and vegetables in general, are largely made up of this starchy or sugary substance, which, as it contains a considerable quantity of carbon, we speak of as the _carbonaceous_ element in food. This is the substance which goes to feed the muscles, replacing the waste from work done, just as fuel is required for the fires of an engine. Yet another substance in food is _fat_. It may be animal, such as beef or mutton fat, and butter, or vegetable, as the oils in nuts, in the olive, etc. Fat, like carbonaceous food, also goes to feed the muscles, but both are required in a healthy diet. Of the first importance, however, is the _proteid_ element in food. Meat, milk, cheese, eggs, peas, etc., contain proteid in considerable quantities. Its use is to repair the exhausted tissues themselves. The muscles and nerves get worn out in their daily work, and require rebuilding. This is what proteid goes to do, and from this, its high import in animal economy, is called Proteid (protos--first). Finally, in all natural foods there are certain _salts_, which also build up, _e.g._, lime, which goes to make up bone. These salts may be seen in the ash of any common vegetable after being burnt. These four kinds of food substance make up our daily food, and a certain amount of each substance is required to replace the daily expenditure, a proportion which varies, however, under different circumstances. _See_ Food in Health. As the relative amount of proteid, carbonaceous matter, water, and salts, may vary considerably in different articles, we rightly have combinations of food at our meals. A pudding of corn-flour and water contains no building material, hence we add milk and eggs, which do. A meal of meat and cheese requires bread and potatoes, etc., etc.