describe the abundance with which they occur.[11] ~Function of "Fat Soluble A."~--All investigators agree that the "A" vitamine is an essential factor in the growth of young tissue, and the repair of mature tissues. McCollum claims that this vitamine is likewise a factor in the prevention of the eye disease known as xerophthalmia, and other scientists also hold this opinion. Eddy states that a diet lacking in the "A" vitamine will, in the majority of cases, result in stunted growth and the development of the eye disease, and that the appearance of the latter may be taken as a sure indication of the absence or deficiency of this vitamine. The following diagram shows the effect of adding fat soluble "A" to the diet which was adequate in other respects. This chart represents the growth curve of young rats.[12] 230 +----------+----------+ | | | | | | | | | | | | | | | 190 +----------+---------/+ | | / | | | / | | | / | | | / | | | / | 150 +----------+---/------+ | | / | | With | / | | "Fat |/ | | Soluble / | | A" /| | 110 +--------/-+----------+ | / | | | / | | | / | | | / | | | / | | 70 +--/-------+----------+ | / | | |/ Without "Fat Soluble A" |----------|----------| | | | | | | 30 +----------+----------+ 0 1 2 Figure showing the effect upon growth of adding "fat soluble A" to a diet adequate in all other respects. Courtesy of Dr. E. V. McCollum. Mellanby of England believes the "A" vitamine to be a factor in the prevention of rickets. Scientists of America have recently investigated this disease, and Dr. Hess (New York) has found cod liver oil to be a remedy for it. Cod liver oil is known to be rich in "Fat soluble A," but whether the cure of rickets is due to the presence of this vitamine in the oil, or to a possible fourth vitamine, is still undetermined. ~Effect of Heat on the "A" Vitamine.~--Heat, as applied in the ordinary methods of cooking, is not believed to exert a great deal of destruction upon the "A" type of vitamine; but hydrogenation, the process used in the hardening of certain fats in the manufacture of lard substitutes, is said to destroy it completely.