Desirable substances fall on the whole into two classes. The first are desirable on account of their physical or chemical properties, for example iron, wood or glass, which we use as a part of systems such as fires, houses, or razors, which procure us certain benefits. The second are desirable on account of their physiological properties. Such substances include foods, drinks, tobacco, and drugs. Colours and scents occupy an intermediate position. The value of this second class of substances rests on a quite special relationship to the human organism which depends in the most intimate way on the constitution of the latter, and has not in general been at all fully explained in terms of physics and chemistry. For example fires can be made of coal or peat instead of wood, but no other chemical substance has the same effect as water or alcohol. So unless a chemical substance has new physiological properties its production will merely serve to improve or make possible some appliance whose use lies within the sphere of applied physics. Within historical time two and only two substances of the second class have come into universal use in Europe, namely caffeine and nicotine, which were introduced into this country in the sixteenth and seventeenth centuries. There are others of immense importance, such as chloroform and quinine, but their use is not universal. But coffee, tea, and tobacco, with alcohol, are as much a part of normal life as food and water. There is no reason to suppose that the list of such substances is exhausted. During the war Embden[1] the professor of physiology in Frankfurt University discovered that a dose of about 7 grams of acid sodium phosphate increases a man’s capacity for prolonged muscular work by about 20%, and probably aids in prolonged mental work. It can be taken over very lengthy periods. A group of coal-miners took it for nine months on end with very great effect on their output. It has no after-effects like those of alcohol, and one cannot take a serious overdose as it merely acts as a purgative. (They gave certain Stosstruppen too much!) Thousands of people in Germany take it habitually. It is possible that it may become as normal a beverage as coffee or tea. It costs 1/9 per pound, or ⅓d. per dose.