"Let us now enumerate some of these fuel-foods. First among them must stand all fat and oily matter of every kind. This, of course, does not surprise you, because these substances are so much like the fuel-food of the egg and milk. I have next some sugar in this spoon. I am going to hold the spoon over the flame of the spirit-lamp. You notice that the sugar soon becomes heated and burns. It is readily combustible and, in burning, gives out great heat.
"We take sugar as a food, because of this. It is a heat- giving or fuel-food. It burns in our bodies and, like fat, is a source of heat and vital energy.
"You remember, no doubt, our early lessons on starch, and you can tell me what kinds of food contain starch.""Yes, sir," replied Fred. "Starch is found more or less in all vegetable foods. It is an important part of thesubstance of the corn-grains, peas, beans, rice, potatoes, arrowroot, sago, and tapioca. I remember how we used to separate the starch from flour by washing and kneading it in the muslin bag under water.""Quite right, Fred. But I wonder whether you remember what happens to these starchy parts of our food when we take them into the mouth.""Oh yes, sir," said Fred eagerly, for the whole thing flashed through his mind like lightning. "The saliva in the mouth changes these starches into sugar. Now I see it all. Starch is another of the fuel-foods. The saliva changes it into sugar, and this sugar burns in the body to provide thebodily heat.
"I remember, you mixed some starch into a paste with water, and made me hold some of it on my tongue for a few minutes, till it began to taste quite sweet. It was the saliva from my tongue that changed the starch into sugar." "Excellent," replied Mr. Wilson, "and now just one thought more, Fred, and we will leave this subject for the present. I am going to take your mind back to that bone which I soaked in the acid. Part of the bone was left-the ossein-but all that had made it hard, firm, and rigid had been dissolved out by the acid. The bones of the little bird, as it was forming in the egg-shell, and those also of the young mammal for some time after its birth, consisted at first entirely of this one substance-ossein. There was, however, stored up in the egg and in the milk, in additionto the substances we have mentioned, a sufficient quantity of earthy matter in each case to change that ossein into actual bone. It could not have become bone without this earthy matter. From the time that the young animals begin to seek their own living, in their own way, Nature supplies the necessary amount of this earthy or mineral matter in the very food they choose.
"If we burn a carrot, a cabbage, or a potato, we find that, although the greater part of its substance is consumed in the burning, there is a residue which will not burn away. It forms an ash. This ash is earthy or mineral matter, which the vegetables took up while they were growing in the ground. Whenever, therefore, we and animals in general eat vegetable food, we take in more or less earthy matter, which those vegetables themselves have absorbed from the soil.
"The very water we drink contains dissolved minerals; salt is a mineral; all our fresh vegetables contain large supplies of mineral matter."