236 



"WELLS'S NATURAL PHILOSOPHY. 



„ . J . 539. As the temperature is lowered, the mercury in Fah- 



great Intensity renheit's thermometer gradually sinks, until it reaches a point 



indicated? ggo below zero, where it freezes. Mercury, therefore, can not 



be made available for measuring cold of a greater intensity. This difficulty 



is, however, obviated by using a thermometer filled with alcohol colored red, 



as this fluid, when pure, never fi-eezes, but will continue to sink lower and 



lower in the tube as the cold increases. Such a thermometer is called a 



spirit thermometer. 



_ . ^ , , 540. If a Fahrenheit's thermometer be heated, the mercury 



How IS neat of "^ 



great intensity contained in it will rise in the tube until it reaches 660°, at 



nieasured ? ■v\-liieh temperature it begins to boil. A slight additional heat 



forms vapor sufficient to burst the tube. Mercury, therefore, can not be used 



to measure degrees of heat of greater intensity than 660° F. Temperatures 



greater than this are determined by means of the expansion of solids ; and 



instruments founded upon this principle are commonly called pyrometers. 



Fig. 205. 



Fig. 206. 



The construction of the pyrometer is represented in Fig. 

 construction of 205 A represents a metallic bar, fixed at one end, B, but 

 the pyrometer. |g{^ f^ee at the other, and in contact with the end of a pointer 

 K, moving freely over a graduated scale. If the bar be heated by the flame 

 ot alcohol, the metal expands, and pressing upon the end of the pointer, moves 

 it, in a greater or less degree. In this manner, the effect of heat, applied for 

 a given length of time, to bars of different metals, having the same length and 

 diameter, may be determined. 



541. The first thermometer 

 air-thermome- used consisted of a column of 

 **'' * air confined in a glass tube over 



colored water. Heat expands the air and in- 

 creases the length of the column downward, 

 pushing the water before it : cold produces a 

 contrary effect. The temperature is thus indi- 

 cated by the height at which the water is ele- 

 vated in the tube. Fig. 206 represents the prin- 

 ciple of the construction of the air- thermometer. 



