The usefulness of soup-material lies, however, not always in its great nutritive capacity, but very often in the facility with which it may be cooked. Thus we cannot boil rice in the broth itself; it must, to loosen its cells properly, be boiled first in water; this takes a little over half an hour, and requires of course a place on the fire, and hence more fuel. The cell of the farina or pearl-barley, on the other hand, was crushed already by the grinding; therefore it needs but little attention, and may be boiled in the broth itself without any loss of time. When making scientific observations on food, such circumstances must not be overlooked; for time and fuel cost money, and may, in the eyes of practical housewives, raise the price of the article too much; while to a scientific man the same article may appear very cheap. There are other viands which, though not very nutritive, are yet very popular and in common use. As an example of this class, we may give the potato. That the latter is poor in starch, was stated above. Its extensive use is surprising, when we consider, that, according to calculation, the little nutriment obtained from the potato is paid more highly for than that of flour. And yet there is good reason for the extensive use of the article. Its preparation, in the first place, is an easy one, especially when the potato is boiled whole, without being peeled. This is a great convenience for the housewife, who, besides the time devoted to the house, needs time for work from the proceeds of which she may support herself. She values, therefore, any dish which can be prepared with little expense of time and money; more than any other article may the potato be said to possess this quality. From it a meal can be prepared in half an hour, and the cook need not watch it constantly; potatoes do not boil over. Besides all this, there is another advantage, and it is this which makes it a favorite even with the rich; already, when boiling, its starch is transformed into sugar, giving the potato a more pleasant flavor than any other cheap dish can be said to have. How easily the potato starch is converted into sugar may be noticed best in half-frozen potatoes, because there the cells containing the starch burst during the process of freezing. CHAPTER XV. LEGUMINOUS VEGETABLES. The greens which we put in soup cannot be considered nutriment, but rather a kind of spice, and perhaps also as a means of giving us the benefit of some medicinal qualities which they in part contain. We will dwell no longer on this subject, but proceed to the most nutritive articles of food we use, viz., the leguminous vegetables.