Certain facts about vitamin D are widely known. It occurs only rarely in foodstuffs. It is formed in the skin by exposure to ultraviolet rays from the sun or from artificial sources. It is developed in some foodstuffs by their being briefly irradiated. It is produced by irradiating ergosterol, the sterol of fungi. From irradiated ergosterol it can be isolated and crystallized.
The fact that vitamin D is not a single chemical substance has only recently been recognized. The erroneous view still commonly held is that ergosterol is the parent substance, or provitamin, from which all vitamin D arises. Ergosterol exhibits four spectral absorbtion bands in the ultraviolet region. These bands were observed in the unsaponifiable fraction of the various materials that become antiricketic on irradiation. Since it is rare that even one band, not to mention such a series of bands, is exactly duplicated by different chemical substances, the