Page:The fairy tales of science.djvu/191

Rh air can possibly enter. As the tube and mercury in it gradually cool, the enclosed fluid contracts and consequently sinks, leaving above it a vacant space or vacuum, through which it may again expand on the application of heat.

To such a tube it is necessary to add a scale, showing at what height the mercury will stand at any given temperature,—for a tube of mercury without a scale would be just as useless as a balance without weights. Now, to form a scale that shall agree with other scales we must find two fixed points, and then divide the intervening space into a given number of equal parts, or degrees. These fixed points are the temperatures of melting snow or ice, called the freezing-point, and of pure boiling water, named the boiling-point. The first is found by plunging the instrument into melting ice, and then, after the temperature of the bath is attained, marking the position of the mercury upon the tube; it is now placed in a deep metallic vessel nearly filled with water, which is heated until rapid ebullition ensues, and in this manner the position of the boiling-point is ascertained. Fahrenheit's scale being the standard generally adopted in England, it is usual to divide the space between the two points into 180 degrees, the freezing-point being marked 32°, and the boiling-point 112°. In the Centigrade thermometer, which is used on the Continent, the space is divided into 100 equal parts, the two points being marked respectively 0°