26. The bread stuffs, wheat, corn, and rye flours, are more than one-half starch. Rice, which is the "staff of life" to one-third of the human family, contains eighty per cent. Unripe fruits have much starch in them, which renders them indigestible when eaten uncooked; for the grains of raw starch are but slightly acted upon within the body. But, under the potent chemistry of the sun's ray, this crude material is converted into sugar. Thus are the fruits prepared by the careful hand of Nature, so that when ripe they may be freely used without further preparation. [Sidenote: 27. Gum, where found? Its composition? Gum Arabic?] 27. GUM is commonly found in those articles which {62} also contain starch; and has the same chemical composition as the latter, but is much less nutritious. In the East, gum-arabic and similar substances are largely employed as food. Persons who travel by caravan across vast, sandy deserts, find such substances well adapted to their wants, since they are not perishable, and are easily packed and carried. [Sidenote: 28. The three classes of food principles? What besides? What is said of them? Name the articles not nutritious.] 28. STIMULATING SUBSTANCES.--The three classes of food-principles already considered--the Albuminoids, the Fats, and the Sugars--comprise all the more important organic ingredients of our food. There are, besides, a great variety of coloring and flavoring matters that stimulate or increase the appetite for food by appealing to the eye and taste; but they are not nutritious, and are quickly separated from the truly useful substances, and do not long remain in the body. Among these may be classed spices, flavors of fruits, tea, coffee, and vegetable acids. [Sidenote: 29. What is said of experiments that have been tried?] 29. NECESSITY OF A REGULATED DIET.--A great variety of experiments have been tried in order to test the relative value of the different nutritive principles. They have been practised to some extent upon man, but chiefly upon those inferior animals which require a similar diet to man. [Sidenote: 30. What has been demonstrated in the first place? Example? Second demonstration? Example? Give the illustration in relation to convertibility.]