Page:Elementary Text-book of Physics (Anthony, 1897).djvu/223

§ 189] the expansion of mercury. The vessel which he used was a glass bulb furnished with a capillary tube. It was filled with mercury at a known temperature, and its volume determined by the weight of the mercury contained in it and the specific gravity of mercury. It was then heated to another known temperature, and the mercury which ran out was collected and weighed. From these data the apparent expansion of mercury in glass could be determined.

When the absolute expansion of mercury is known the knowledge of its apparent expansion in glass enables us to determine the absolute expansion of glass also.

If the apparent expansion of mercury be known, and if we assume that its expansion is proportional to the rise of temperature, we may evidently use the amount of mercury which runs out when the bulb is heated as a measure of its change of temperature. The instrument just described is therefore called a weight thermometer.

188. Determination of Voluminal Expansion of Solids.—The weight thermometer may be used to determine the coefficient of voluminal expansion of solids. For this purpose, the solid, of which the volume at zero is known, must be introduced into the bulb by the glass-blower. If the bulb containing the solid be filled, with mercury at zero, and afterward heated to the temperature $$t,$$ it is evident that the amount of mercury that will overflow will depend upon the coefficient of expansion of the solid, and upon the coefficient of apparent expansion of mercury. If the latter has been determined for the kind of glass used, the former can be deduced. By this means the coefficients of voluminal expansion of some solids have been determined; and the results are found to verify the conclusion, deduced from theory (§ 185), that the voluminal coefficient is three times the linear.

189. Absolute Expansion of Liquids other than Mercury.—The weight-thermometer may also serve to determine the coefficients of expansion of liquids other than mercury; for, if the absolute expansion of glass has been found as described above, the instrument may be filled with the liquid the coefficient of which is desired, and the