If we are making up the balance-sheet of a fully-grown man, we may take for granted that he is not making fresh tissue. During the period throughout which he is under observation, care is taken to avoid altering the conditions of his life in such a manner as to lead him to develop additional muscle. If he gains in weight while under observation, he is putting on fat. If he loses in weight, he is sacrificing fat. The whole of the nitrogen taken in leaves the body in urea, unless, as we have said, growth of tissue is taking place. The body has not the same temptation to store nitrogen as it has to store carbon. Consequently, it is very sensitive to any deficiency of nitrogen in the diet. If food does not contain as much protein as is needed, the deficit is made up at the expense of the tissues. It does not necessarily follow that under these circumstances a man loses in weight. He may be putting on fat, although losing in strength owing to waste of muscle. For observations upon the income and expenditure of the body to be of any value, a condition of “nitrogenous equilibrium” must be established. The nitrogen taken in must equal in amount the nitrogen given out. Very exact determinations of income and expenditure may be made by placing an animal, or even a man, in a box through which air is drawn. A record is made of the volume of air drawn through the box. The percentages of water vapour and carbonic acid which the air contains are estimated before it enters and after it leaves. The solid food consumed and the urea excreted are also measured. If it is desired to measure the amount of heat given off, an animal may be placed in a calorimeter. Even when most passive, the subject under examination, whether an animal or a man, is expending energy in keeping the body warm, in movements of respiration, and in shifting position. If it is desired to ascertain the relation of oxidation to external work, it is easy to devise a form of resistance, such as the turning of a wheel, or the lifting of a weight which can be measured. In testing diets, it suffices to make sure that nitrogenous equilibrium is maintained, and then to estimate the gain or loss in weight and the output of energy in external work.