Experimental cookery

The fact that it is harder for the vapor to leave the surface of the liquid when there is a soluble substance in it is made use of in the following or similar ways to keep food moist. A covered vessel containing food loses moisture from the food until the air space is saturated with vapor. If the vessel is tight enough, the food does not dry out to an appreciable extent. If two dishes are placed in an enclosed vessel, one of them containing water and the other a heavy sugar solution, the sugar solution, since it loses vapor with difficulty and absorbs liquid, will gradually absorb water from the other dish. If two foods are placed in the same container, the one with more sugar will gradually absorb moisture from the other. This is why an apple is often put in a box with a fruit cake, the apple drying 46 SUGAR COOKERY up and keeping the cake moist. Cake and bread should not be stored to- gether, as the bread will become dry quickly. The Boiling Point of Water and Solutions Water standing in an open vessel gradually evaporates, so that eventually all the water disappears in the form of vapor. We know that evaporation takes place more rapidly on warm days than on cool ones, and more rapidly from a wide shallow vessel than from a narrow deep one. As the tempera- ture is increased, the rapidity of the motion of the molecules is increased. Therefore a larger number escape from the liquid as the temperature increases, most of them being carried away by air currents. Saturated vapor. If we cover a vessel of water, leaving an air space between the surface of the liquid and the cover, evaporation takes place for a time just as from an uncovered vessel. But when the vapor cannot be carried away, the air above the liquid soon becomes filled with vapor mole- cules. Of course some of them reenter the liquid. When they are entering the surface of the liquid as rapidly as they are leaving it, the air is said to be saturated and is in equilibrium with the liquid, i.e., the saturation point is reached when the air holds all the vapor possible at that temperature. If the temperature is increased the velocity of the molecules is increased, and they leave the surface of the liquid faster than they enter until equi- librium is again established. If the temperature is reduced, a part of the vapor condenses into the liquid and forms drops of liquid on the sides and cover of the vessel or on the surface of the liquid. The boiling point. When water is heated slowly enough, air bubbles are noticed forming on the sides and bottom of the pan. They come from the air that has been held in solution by the water. A similar thing may be noticed on a warm day when a glass or pitcher of cool water is left in a warm room. The air bubbles collect on the sides of the glass or pitcher, and if the vessel is jarred many of them will rise to the top of the water and break. If heating of the water is continued the vapor begins to form. Many of the first vapor bubbles collapse before they reach the surface.