(7) Lift the thermometer above the water and note the temperature just above the surface, when the water is rapidly boiling.
The small bubbles are bubbles of air. The large are bubbles of steam. A complete study of the boiling process should be made in the Physics class. The boiling point is the point at which water becomes steam, and also the point at which steam condenses again to water. The temperature of boiling water and steam are the same. Under pressure steam may be heated to a higher temperature.
B. Boil water in a small saucepan closely covered.
(1) Note the sounds of the water just before boiling, and the change in sound as the boiling begins.
(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.)