These substances are usually defined as compounds of carbon, hydrogen, and oxygen, in which the last two elements are in the same proportion as in water, and in the main this is so. They are essentially vegetable products - starch, cellulose, cane sugar, grape sugar or glucose, fruit sugar (laevulose or fructose) being typical examples. Two are found in the animal body, viz. milk sugar or lactose and glycogen or animal starch, the latter being present in the liver, muscle, white-blood corpuscles and other tissues. The more common forms are divided int othree classes - (1) the monosaccharoses: grape sugar (glucose), fruit sugar (laevulose), and galactose; (2) the disaccharoses: cane sugar (saccharose or sucrose), milk sugar, and malt sugar; (3) the polysac-charoses: starch, dextrin, glycogen, cellulose. It is interesting to note that before they can be utilised in the body as food they must each be converted into grape sugar.

Sugars are all easily soluble in water, and for household purposes are mainly derived from the sugar cane and the sugar beet, in about equal proportions. It is a great mistake to look upon sugar as simply a condiment, as although it is incapable of supporting life on account of its lack of nitrogen, it is a most important nutrient. It produces heat and energy in a much more economical and agreeable form than protein or fat. The food-fruits, apples, pears, grapes, bananas, owe their food value almost entirely to their content of sugar. The great value of sugar as an energy-producing food is now so well known that in the German army whilst on the march a daily ration of four ounces is issued to each man. It is equally useful as a nutrient in the case of children, and in the winter time when given to the limits of a child's digestive capacity its heat-producing power is calculated to save the expense of an extra garment. It is apt, however, to be indulged in to excess, and in this way may injure the child's appetite and digestion. It is stated to have no ill effects on the teeth, but this is rather doubtful, and in any case there is an enormous contrast between the beautiful molars of the sugar-cane-loving negro and the decaying stumps of the sweet-eating Briton.

Sugar can also be obtained from the sap of the maple tree and from corn and other starches. Molasses is a by-product obtained in the refining of sugar, and syrups are obtained from the evaporation of the saccharine juices of plants. Honey is composed of 60-75 per cent. of invert sugar (i.e. derived from natural sugars by inversion) and water, and shares with milk the distinction of being the only dietetic agencies originally intended for this purpose and no other. It is much more easily digested than sugar.

When yeast is allowed to grow in a solution of sugar, alcohol and carbonic acid are produced, and this alcoholic fermentation is well illustrated in the manufacture of beer.

Starch is found in practically all plants, and is composed of a number of overlapping layers separated by cellulose. It is specially abundant in potatoes and cereals. It is insoluble in cold water, but when treated with boiling water the little starch grains swell up and rupture the cellulose envelopes. When heated in the dry state it is converted into dextrin (the common gum used for postage stamps) and soluble carbohydrates. Dextrin is formed in this way in the crust of bread and wherever potatoes or other starchy foods are browned.

Cellulose is the groundwork or skeleton of plants within which the cells are enclosed. It has the same chemical composition as starch, but except in young and tender plants it is quite indigestible, and therefore prevents its nutrient contents from becoming available as food, unless by heat or otherwise the cellulose envelope is ruptured. Carbohydrates and fats are consumed by burning in the air or oxidation in the body, their end-products - when combustion is complete, as even in the body it usually is - being carbonic acid and water.

It is convenient to complete our review of the alimentary principles at this point, and for this purpose we make a short reference to the mineral matters and water.