If we take the mean quantity of 8 ounces, then to supply this alone, a man must eat 18 ounces of starch or sugar every day. If he take it in the form of wheaten bread, he will require 1¾ lbs. of bread, if in the form of potatoes, about 7½ lbs. of raw potatoes, to supply the waste caused by his respiratory organs alone. When the habits are sedentary, 5 lbs. of potatoes may be sufficient, when violent and continued exercise is taken, 12 to 15 lbs. may be too little. At the same time, it must be observed, that where the supply is less, the quantity of carbonic acid given off will either be less also, or the deficiency will be supplied at the expense of the body itself. In either case the strength will be impaired, and fresh food will be required to recruit the exhausted frame. 2. _The food must repair the daily waste of the muscular parts of the body._ When the body is full grown, a portion from every part of it is daily abstracted by natural processes, and rejected either in the perspiration or in the solid and fluid excrements. This portion must be supplied by the food, or the strength will diminish—the frame will gradually waste away. The muscles of animals, of which lean beef and mutton are examples, are generally coloured by blood, but when well washed with water, they become quite white, and, with the exception of a little fat, are found to consist of a white fibrous substance, to which the name of _fibrin_ has been given by chemists. The clot of the blood consists of the same substance; while skin, hair, horn, and the organic part of the bones, are composed of varieties of _gelatine_. This latter substance is familiarly known in the form of _glue_, and though it differs in its sensible properties, it is remarkably analogous to _fibrin_ in its elementary constitution, as both of these substances are to the white of the egg (_albumen_), to the curd of milk (_casein_), and to the _gluten_ of flour. They all contain nitrogen, and all consist of the four elementary bodies (organic elements), very nearly in the following proportions:— Carbon, 55 Hydrogen, 7 Nitrogen, 18 Oxygen, 20 ---- 100 They all contain, likewise, a small proportion of sulphur and of phosphorus. The quantity of one or other of these removed from the body in 24 hours, either in the perspiration or in the excretions, amounts to _about five ounces_, containing 350 grains of nitrogen, and this waste at least must be made up by the gluten or fibrin of the food. In the 1¾ lb. of wheaten bread we have supposed to be eaten to supply carbon for respiration, there will be contained also about 3 ounces of gluten. Let the other 2 ounces be made up in beef, of which half a pound contains 2 ounces of dry fibrin, and we have