As a result of these experiments McCollum and Davis (22) formulated in 1915 their working hypothesis of what constitutes an adequate diet. The diet must contain, in addition to the long recognized dietary factors, viz., protein, a source of energy in the form of proteins, carbohydrates and fats; a suitable supply of certain inorganic salts, two as yet unidentified substances or groups of substances. One of these, fat soluble A, is associated with certain fats, and is especially abundant in butter fat, egg yolk fats, cod liver oil and the fats of the glandular organs such as the liver and kidney, but is absent or present in but traces in fats or oils of vegetable origin. The second substance, water-soluble B, is never associated with fats or oils of either animal or vegetable origin. It is widely distributed in natural foods, and can be isolated in a concentrated, but not in a pure form, from natural food-stuffs by extraction with either water or dilute alcohol. This water or alcoholic extract always contains the substance which cures polyneuritis.