The chemical composition of the potato tuber varies greatly according to the conditions under which it has been grown, namely: soil, weather, manure, etc. It contains about seventy-five per cent. of water. The composition of the twenty-five per cent. of dry substance is as follows: protein, 2 per cent.; fat, .03; starch and other carb-hydrates, 20.7; fibre, 1.1; ash, .09. By protein is understood the various compounds containing nitrogen, like the gluten of wheat, white of egg, etc. This is considered the flesh-forming part of a food. Lean flesh is made up largely of protein compounds, or albuminoids, as they are sometimes called. The carb-hydrates contain no nitrogen, and are compounds of carbon, hydrogen and oxygen. Among the most familiar of this class are starch, sugar, and gums. The carb-hydrates, in contrast with the flesh-forming protein compounds are frequently called heat producing substances. This classification aids in giving a general idea of the part the two groups play in the animal economy. Both classes of foods are required, the amount of each depending upon the wear and tear of the body and the conditions of temperature, etc., under which the animal lives. A man who is working hard needs more of the protein compounds to build up the muscles than the person of leisure. When exposed to severe cold, an increase of the starchy substance is demanded to make good the losses caused by the excess of animal heat produced. From the average of many chemical analyses given above, it is seen that the potato is a heat-producing food, and not a muscle-forming food. The fat in foods—of which there is very little in the potato—is used both as fuel and to build up the fatty substance of the body. The proteins or albuminoids are the most expensive portions of any food; the fats come next, and the carb-hydrates last. (In this way a chemist is able to compute the nutritive value of a food from the per cent. of the classes of constituents found present by analysis.) Wheat contains about eleven per cent. of protein, and seventy per cent. of carb-hydrates. There is far less water, but more than that, it has a higher ratio of protein. It is a richer food. Corn has the same per cent. of starchy matter, but only nine per cent. of the albuminoids. It is not so rich a food as wheat. Beans and peas have about fifty per cent. of oil and starch and twenty-five per cent. of the flesh-formers. This is a very high protein ratio, and those grains approach closely to the composition of meats, and may replace them to a large extent. This is all to show that both chemistry and culinary experience do not rank the potato as a rich food. It serves the animal economy best when eaten with some other substance far richer in protein. Thus we have meat and potatoes as a wholesome and complete food.