_Sulphur, Iron, Soda_, and _Potash_ are all necessary in the various tissues of the body, and deficiency of any one of them, for any considerable length of time, results in disease. They are all supplied, variously arranged and combined, in both animal and vegetable food; in some articles they exist to a considerable extent, in others in much smaller quantities. _Sulphur_ exists in eggs and in the flesh of animals, and often in water. _Iron_ exists in the yolk of eggs, in flesh, and in several vegetables. _Soda_ is supplied in nearly all food, and largely in common salt, which is a composition of sodium and hydrochloric acid, the latter entering into the gastric juice. _Potash_ exists, in some form or other, in sufficient quantities for health, in both vegetable and animal food. CLASSES OF FOOD. All kinds of food substances may be divided into four classes. _Proteids, Fats, Amyloids_, and _Minerals_. Proteids are composed of the four elements, carbon, hydrogen, oxygen, and nitrogen, sometimes combined with sulphur and phosphorus. In this class are included the _gluten_ of flour; the _albumen_, or white of eggs; and the _serum_ of the blood; the _fibrin_ of the blood; _syntonin_, the chief constituent of muscle and flesh, and _casein_, one of the chief constituents of cheese, and many other similar, but less frequent substances. Fats are composed of carbon, hydrogen, and oxygen only, and contain more hydrogen than would be required to form water if united with the oxygen which they contain. All vegetable and animal oils and fatty matters are included in this class. Amyloids consist of substances which are also composed of carbon, oxygen, and hydrogen only; but they contain just enough hydrogen to produce water when combined with their oxygen, or two parts of hydrogen to one of oxygen. This division includes _sugar, starch, dextrine_, and _gum_. The above three classes of food-stuffs are only obtained through the activity of living organisms, vegetable or animal, and have been, therefore, appropriately termed by Prof. Huxley, _vital food-stuffs._ The mineral food-stuffs may, as we have seen, be procured from either the living or the non-living world. They include water and various earthy, metallic, and alkaline salts.