as to the exact amount of the different amino-acids yielded by the digestion of the proteins even of many of the common articles of food is not available. But many workers are investigating these matters, and the next great advance in the science of dietetics will probably come along these lines. By almost universal custom certain articles of food are commonly eaten in association: bread and cheese, eggs and bacon, are instances. Such customs are usually found to be based on some underlying principle. The principle in this case may well be that of complementary proteins. The remarks which have been made above on the subject of the _rôle_ of protein in the animal economy apply to adults in which protein is required for wear and tear only and not for increase in weight. They will obviously apply with greatly increased force to the case of growing children, who require protein not only for wear and tear, but for the building up of their muscles and other working parts as they grow and develope. Consequently the diet of children should contain more protein in proportion to their size than that of adults. For this reason it is not desirable that bread should form an excessive proportion of their diet. The bread they eat should be supplemented with some other food richer in protein. The ash of bread although so small in amount cannot be ignored, in fact it is regarded as of very great importance by modern students of dietetics. The particular constituent of the ash to which most importance is attached is phosphoric acid. This substance is a necessary constituent of the bones and of the brain and nerves of all animals. It exists too in smaller proportions in other organs. Like other working parts of the body the bones and the nervous system are subject to wear and tear, which must be replaced if the body is to remain in normal health. A certain daily supply of phosphoric acid is required for this purpose, and proportionally to their size more for children than for adults. Considerable difference of opinion as to the exact amount required is expressed by those who have investigated this question, nor is it even agreed whether all forms of phosphoric acid are of the same value. There is however a general recognition of the importance of this constituent of the diet, and the subject is under investigation in many quarters. CHAPTER VIII CONCERNING DIFFERENT KINDS OF BREAD