Fats are _stored away_ as fat, _furnish heat_, and are _used for energy_; at least, it is probable that at times they are put to the latter use. The fats laid by in the body for future use last in cases of starvation quite a long time, depending, of course, upon the amount. At such times a fat animal will live longer than a lean one. Doubtless in the fat of food the body finds material for its fats in the most easily convertible form. Of the various fatty substances taken, some are more easily assimilated than others. Dr. Fothergill, in "The Town Dweller," says that the reason that cod-liver oil is given to delicate children and invalids is, that it is more easily digested than ordinary fats, but it is an inferior form of fat; the next most easily digested is the fat of bacon. When a child can take bread crumbled in a little of this fat, it will not be necessary to give him cod-liver oil. Bacon fat is the much better fat for building tissues. Then comes cream, a natural emulsion, and butter. He further says there is one form of fat not commonly looked at in its proper dietetic value, and that is "toffee." It is made of butter, sugar, and sometimes a portion of molasses. A quantity of this, added to the ordinary meals, will enable a child in winter to keep up the bodily heat. The way in which butter in the form of toffee goes into the stomach is particularly agreeable. =Carbohydrates.= The principal carbohydrates are _starch_, _dextrine_, _cane-sugar_ or common table sugar, _grape-sugar_, the principal sugar in fruits, and _milk-sugar_, the natural sugar in milk. They are substances made up, as before stated, of carbon, hydrogen, and oxygen, but no nitrogen. They are important food substances, but are of themselves incapable of sustaining life. The carbohydrates, both starch and sugar, in the process of digestion are converted into glucose. This is stored in the liver in the form of _glycogen_, which the liver has the power of manufacturing; it then passes into the circulation, and is distributed to the different parts of the body as it is needed. (The liver also has the power of forming glycogen out of other substances than sugar, and it is pretty conclusively proved that it is from proteids, and not from fats. Carnivorous animals, living upon flesh alone, are found to have glycogen in their bodies.)