The investigations of Liebig, who first studied this question, and those of his school, demonstrate that certain salts are closely combined with the other elements of living bodies, forming integral parts of them. In all the tissues of the organism is found potash in combination usually with phosphoric acid. In the. blood the salts of soda predominate. When we reduce the organs to ashes, these vary but very little in quality or in quantity. It is unquestionable that without these salts no organ is developed, and that there is no secretion by the glandular cellules. All the secretions contain certain salts, most of them characteristic. These salts have definite functions to discharge in the animal economy. Still, we do not know the quantity of these salts which must enter into the food, to support the body, though this is a highly-important question in the hygiene of alimentation. Direct experiments alone could decide, for Voit shows that those made by Magendie, and which are so frequently referred to, do not establish the points they are supposed to establish. Recent studies upon the nutritive value of gelatine have shown that Magendie failed to take into account some of the principal points of the question he was considering.
"For a long time," says Voit, "it had been my purpose to ascertain by thorough experimentation the value of salts in nutrition, with a view to examining how long an animal could live without them, and what symptoms it would manifest. For this purpose I had accumulated a quantity of the residuum of meat-extract. In the mean time appeared a remarkable work by Kemmerich, in which were proposed other questions, having a bearing, in many respects, upon those I had proposed to myself. Kemmerich starts out with the supposition, which Liebig, too, admits, that the residuum of the meat, without the extract, has no value as nutriment. According to his investigations, the action of meat-extract is attributable to the potash it contains, whence he concludes that the residuum is of no value for nutrition, owing to the absence of the potash. He therefore attempted to utilize it by adding to it the salts contained in meat. To this moist residuum, three times exhausted by boiling, he adds an artificial mixture of the salts of meat and common table-salt; and on this food exclusively he fed two dogs six weeks old, they devouring it ravenously. The experiment was continued for three months, and at the end of that period the dogs had gained considerably in weight."
Kemmerich repeated his experiments on the same animals, giving them now the residuum without the mixture of salts. He now observed that the animals consumed less and less from day to day, even when suffering from hunger. Again, experimenting on two dogs six weeks old, he fed to the one the residuum mixed with the salts; to the other the residuum mixed only with common table-salt. After twenty-one days a wide difference was observable between the two animals. The first one weighed much more than the second, was stronger and more intelligent. The second one had gained a little in weight, it is