20 THE THERMOMETER. 



Now, does this half heat produce a half expansion in mercury ? 

 On trial we find that it does. In the mixture of equal parts of the 

 hot and cold water, the mercury stands exactly half way between 

 the marks, supposing the experiment to be conducted with the 

 proper precautions. This proves that the dilatations of mercury 

 are proportional to the intensity of the heat which produces 

 them. In the mercurial thermometer, therefore, quantities or 

 degrees of expansion may be taken to indicate quantities or 

 degrees of heat ; and that is the principle of the instrument. 



The same correspondence exists between the expansions of 

 air and the quantities of heat which produce them. Indeed in 

 the case of air, the correspondence is rigidly exact, while in 

 the case of mercury it is only a close approximation. Thus 

 Dulong and Petit found that the boiling point of mercury was, 

 as measured by mercury in a syphon . . 680. 



the air thermometer . . . 662. 

 mercury in glass, (Mr. Crichton) 660. 



A short table exhibiting the increasing rate of the expansions 

 of mercury has already been given, but glass expands in a ratio 

 increasing still more rapidly than this metal ; so that the greater 

 expansion of the mercury in the thermometer at high tempera- 

 tures, is fortunately corrected by the increasing capacity of 

 the glass bulb. 



Fixed oils and spirits of wine do not deviate far from uni- 

 formity in their expansions, at least at low temperatures, and 

 therefore are sometimes used as thermometric liquids. 



Thermometers have been devised which indicate the highest 

 and lowest temperature which has occurred between two observa- 

 tions, or are self-registering. Six's thermometer, which was 



invented by Dr. Ru- 

 therford is of this 

 kind. This instru- 

 ment consists, pro- 

 perly speaking, of 

 two thermometers, 

 one 0, of spirit of wine, and the other b of mercury, which are 

 placed in the position represented in the figure, their stems 

 being in a horizontal direction. The thermometer b is intended 

 to indicate the maximum temperature. It contains, in advance 

 of the mercury, a short piece of iron wire, which the mercury 

 carries forward with it in dilating, and which remains in its 



