The actual practical knowledge of how a certain number of dishes should be made has, of course, its value; but it is not the only consideration which should enter into the teaching of cookery. Perhaps the most important point in all such work is the recognition in certain cases of the _necessity_ for particular dishes, and the reasons for, and the value of, their ingredients. Why one kind of food is better for one person and a different kind for another is, without doubt, an essential point in all such study. A system depleted by disease, exhausted by long-continued illness, is an exceedingly delicate instrument to handle. It requires the greatest wisdom and good judgment on the part of physician and nurse to restore a patient to health without a lingering convalescence. There is no doubt that the period of convalescence may be much shortened by the wise administration of food, and that the subsequent health of the patient may be either made or marred by the action of the nurse in this respect. PART I EXPLANATORY LESSONS PART I EXPLANATORY LESSONS PREPARATION OF FOOD =Digestibility.= There are comparatively few kinds of food that can be eaten uncooked. Various fruits, milk, oysters, eggs, and some other things may be eaten raw, but the great mass of food materials must be prepared by some method of cooking. All the common vegetables, such as potatoes, turnips, carrots, beets, and the different grains, such as rice, wheat, corn, oats, etc., neither taste good nor are easily digestible until their starch, cellulose, and other constituents have been changed from their compact indigestible form by the action of heat. Some one has spoken of cooking as a sort of artificial digestion, by which nature is relieved of a certain amount of work which it would be very difficult, if not impossible, for her to perform. =Flavors.= The necessity of cooking to develop, or to create, a palatable taste is important. The flesh of fowl is soft enough to masticate, but only a person on the verge of starvation could eat it until heat has changed its taste and made it one of the most savory and acceptable of meats. Coffee also well illustrates this point. When coffee is green--that is, unbrowned--it is acrid in taste, very tough, even horny in consistency, and a decoction made from it is altogether unpleasant. But when it is subjected to a certain degree of heat, for a certain time, it loses its toughness, becomes brittle, changes color, and there is developed in it a most agreeable flavor. This flavoring property is an actual product of the heat, which causes chemical changes in an essential oil contained in the bean. Heat not only develops but creates flavors, changing the odor and taste as well as the digestibility of food.