The change in temperature of a body is ordinarily measured by thermometers, though for very high temperatures so-called pyrometers are used. The latter are dealt with under the heading "High Temperature Measurements" at the end of this chapter. [Illustration: Fig. 11] By reason of the uniform expansion of mercury and its great sensitiveness to heat, it is the fluid most commonly used in the construction of thermometers. In all thermometers the freezing point and the boiling point of water, under mean or average atmospheric pressure at sea level, are assumed as two fixed points, but the division of the scale between these two points varies in different countries. The freezing point is determined by the use of melting ice and for this reason is often called the melting point. There are in use three thermometer scales known as the Fahrenheit, the Centigrade or Celsius, and the Réaumur. As shown in Fig. 11, in the Fahrenheit scale, the space between the two fixed points is divided into 180 parts; the boiling point is marked 212, and the freezing point is marked 32, and zero is a temperature which, at the time this thermometer was invented, was incorrectly imagined to be the lowest temperature attainable. In the centigrade and the Réaumur scales, the distance between the two fixed points is divided into 100 and 80 parts, respectively. In each of these two scales the freezing point is marked zero, and the boiling point is marked 100 in the centigrade and 80 in the Réaumur. Each of the 180, 100 or 80 divisions in the respective thermometers is called a degree. Table 3 and appended formulae are useful for converting from one scale to another. In the United States the bulbs of high-grade thermometers are usually made of either Jena 58^{III} borosilicate thermometer glass or Jena 16^{III} glass, the stems being made of ordinary glass. The Jena 16^{III} glass is not suitable for use at temperatures much above 850 degrees Fahrenheit and the harder Jena 59^{III} should be used in thermometers for temperatures higher than this. Below the boiling point, the hydrogen-gas thermometer is the almost universal standard with which mercurial thermometers may be compared, while above this point the nitrogen-gas thermometer is used. In both of these standards the change in temperature is measured by the change in pressure of a constant volume of the gas. In graduating a mercurial thermometer for the Fahrenheit scale, ordinarily a degree is represented as 1/180 part of the volume of the stem between the readings at the melting point of ice and the boiling point of water. For temperatures above the latter, the scale is extended in degrees of the same volume. For very accurate work, however, the thermometer may be graduated to read true-gas-scale temperatures by comparing it with the gas thermometer and marking the temperatures at 25 or 50 degree intervals. Each degree is then 1/25 or 1/50 of the volume of the stem in each interval.