In 1914 McCollum and Davis observed that butter fat may be saponified with alcoholic potassium hydroxid and petroleum ether without the destruction of the nutritive principle now designated fat-soluble A or fat-soluble vitamin (25). The soaps resulting from this saponification were dissolved in water and shaken with olive oil. This olive oil had been shown by previous experiments to be ineffective as a substitute for butter fat for the promotion of growth when added to an otherwise complete diet. The olive oil, which had thus been in intimate contact with the soaps of butter fat, was then removed by ether, and the latter evaporated. This olive oil was found by appropriate experiments to have acquired the nutritive properties of the butter fat, since it caused a marked response with growth in rats which had been denied the dietary essential fat-soluble A, and whose growth had been for that reason suspended. This experiment showed that the dietary essential in butter fat is sufficiently stable to withstand saponification in nonaqueous solutions. Steenbock, Sell and Buell (26) have since recorded as an original observation their findings that fat-soluble vitamin resists saponification in the cold by alcoholic potash.

The foregoing account of the investigations which have been directed toward discovering methods for the isolation and purification of the dietary essential fat-soluble A, have actually yielded only the information that the substance is thermostable in the absence of oxygen, that it will withstand the action of saponifying agents in the absence of water, and that after saponification it accompanies the carotin fraction of the plant pigments rather than the xanthophyll. We have no knowledge concerning the chemical nature of the substance, and it is to be hoped that the near future will bring important results in the direction of revealing its properties and its chemical composition.