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deg, degrees, mercury and freezing

TEST TUBE.—A thin glass tube with a rounded bottom used in making tests. See fig. 464. TEST TUBE-HOLDER.—An arrangement for holding one or more test tubes. See fig. 465.

THERMOMETER.—An instrument for measuring intensity of heat or temperature by means of expansion of a liquid or gas. Mercury is generally .employed, and an ordinary ther mometer consists of a spherical or cylindrical glass bulb at the end of a very fine tube, the bulb being completely filled, and the tube partly filled with mercury, while the space above the mer cury contains only a small quantity of mercury vapor, which offers no resistance to the expansion of the mercury. When the temperature rises the mercury expands, and consequently rises in the tube. On the other hand, a fall in the temperature is indicated by a fall of the mercury in the tube. A graduated scale is attached with two fixed points, the freezing point and the upper or boiling point of water. The distance between the two fixed points is then divided into a number of degrees, which are continued above and below the two fixed points. In the Centigrade or Celsius thermometer, used by scientists all over the world, and in general use on the Continent, the distance between the two points is divided into a hundred degrees, the freezing point being o deg. and the boiling point ioo deg.

In the Reaumur scale the distance is divided into 8o deg., the freezing point being o deg.,

and the boiling point 8o deg.

On the Fahrenheit thermometer, which is the one most generally used in this country, the freezing point is marked 32 deg., and the boiling point 2 1 2 deg:, hence the zero of the scale on that part marked o deg. is 32 degrees below the freezing point, and the interval between the freezing and the boiling points consists of 18o degrees. The zero point is supposed to have been fixed by Fahrenheit at the point of greatest cold he had observed.

Degrees above o deg. are termed + degrees, and those below o deg. are termed — degrees. The following formula will serve to convert any given number of degrees of one scale into the corresponding number of one of the other scales :— For extreme degrees of cold mercury cannot be employed, as it freezes at about 4o degs. below zero on the Fahrenheit scale. They can only be used for temperatures between — 4o degs. and + 675 degs., as it boils at the latter point. For lower temperature alcohol is used, as this has never been known to freeze, and for higher temperatures air thermometers are employed, in which changes of temperature are measured by the contraction of a known volume of air. Alcohol cannot be used for high temperatures, as it is soon converted into vapor. See fig. 466.