Four rats per set. Weight on first day:

A 255 B 254

For three days both A and B were fed on the regular diet. At the end of the third day:

A 260 B 261

B were now fed on milk alone. On the 43 day:

A 293 B 258

With the adult rat the shortcomings of an exclusive milk diet are far more obvious than with the young rat. It is possible that the animal did not receive enough total solids; or, to put it in another way, the milk contained too much water and too little solids.

The Effect of a "Synthetic Diet" Those who have read the account of Prof. Hopkins' experiment (page 91) will readily understand what is meant by a "synthetic diet." The "diet" used in the experiments to be described consisted in the proportions (in grams) of casein 25, starch 53.5, lard 24, salt mixture 4, and water 300 cubic centimeters (1000 cubic centimeters correspond to one liter, and this in turn is the approximate equivalent of a quart). The salt mixture was composed of tri-basic calcium phosphate 10, potassium mono-hydrogen phosphate 37, sodium chloride 20, sodium citrate 15, magnesium citrate 8, calcium lactate 8, and ferric citrate 2.

The casein used was carefully purified by repeated extraction with ether (which had been prepared by redistillation over sodium). This is necessary to free it from any adhering vitamine material, particularly the vitamine A (or the fat-soluble vitamine). Recent experiments by the author in another connection have confirmed Drummond's assertion that the vitamine adhering to the casein can be equally well destroyed by heating the casein in a shallow dish around 105 degrees centigrade for 24-48 hours, being careful to stir the mass thoroughly from time to time, and to have as thin a layer of the protein as possible, so as to offer every facility for oxidation. The effect of oxidation on vitamines will be discussed later.

For this experiment 5 rats per set were selected. Weight on the first day:

A 63 B 67

For five days both A and B were fed on the regular diet. At the end of the 5 day:

A 77 B 85

B were now fed on "synthetic diet" alone. On the 51 day:

A 206 B 130

B were now fed on the regular diet. On the 96 day:

A 253 B 245

These figures are worth a little further analysis. The record of the experiment shows that the B animals, while on the synthetic, continued to gain until the 33 day, when their weight (average) was 135. It is true, however, that even up to this time the control animals gained more rapidly, for their weight on the 33 day was 170. From the 33 to the 55 day the B animals remained more or less stationary, showing a decline towards the end, when their weight dropped to 130. At this point the regular diet was substituted for the "synthetic," and within 6 days - from the 51 to the 57 - the B animals gained 45 grams (from 130 to 175). From then on they made rapid, though not quite such remarkable strides, and by the end of the experiment they were fully the equal of the controls.

This experiment clearly shows the importance in the diet of substances other than protein, fat, carbohydrates, and mineral matter (salts).