(2) Note the difference between the vapor escaping, before boiling, and after. This experiment is best performed in a teakettle. _C._ Test the temperature of the inner part of the double boiler, when the water boils rapidly below. To be exact, a hole should be bored in the cover of the boiler, a cork with a hole inserted, the thermometer run through the cork. An approximate result is obtained by putting in the thermometer, setting on the cover tilted, and covering the opening with a cloth. _D._ Stir salt into rapidly boiling water in the lower part of the double boiler until no more salt will dissolve (a saturated solution). Test the temperature. _E._ Put the inner part of the double boiler containing water into this boiling solution of saturated salt, being sure that the inner part is sufficiently deep in the salt solution. Note the temperature of the water in the inner boiler when it becomes heated. =Boiling at high altitudes.=——When the air pressure upon the surface of the water is lessened, the water boils at a lower temperature. As the altitude increases, the air pressure decreases, as many a mountain traveler knows to his cost. The boiling temperature of water is so much lowered that the dwellers in high regions of several thousand feet find it difficult to cook starchy vegetables well. A heavy iron pot is made with clamps for fastening down a tight cover, which increases the temperature somewhat. Experiments _D_ and _E_ indicate a method that can be used to a small extent. The baking process should be largely used, and boiling avoided. For meat, eggs, and fish the lower temperature is not undesirable. (See the chapters relating to these foods.) =The uses of ice.=——Water freezes and ice melts at the same point, 32° F., or 0° C. If ice is mixed with salt, the temperature is reduced far below the freezing point, nearly to 0° F. This process reduces any watery substance which it surrounds to its freezing point, the heat being used in the melting of the ice. This is an interesting topic to discuss in the Physics class. Ice at its ordinary temperature of 32° F. is used for cooling food agreeably. Its most important function in the refrigerator is as preserver of food for a short time at least. For this it is invaluable, and cheap ice is really necessary in summer to the health of a great city. =Ice substitutes.=——Where the supply fails or the price is exorbitant, one property of water makes it a partial aid. The rapid evaporation of water will absorb heat so rapidly as to reduce the temperature of adjacent bodies. In the tropics when ice is lacking, water is hung in porous jars in the breeze, and the temperature of the water in the jar is reduced. To keep milk and butter cool wrap a wet cloth about the containing jar, and set the jar upon the window sill, keeping one end of the towel in a vessel of water; or the cloth may be wrapped directly around the butter. This method is surprisingly effective.