142 GENERAL SCIENCE 



mercury in them at the boiling and freezing temperatures of 

 water, even when divided into 100 (or 180) equal parts, 

 gives spaces (degrees) sufficiently long to be easily read. 

 Fractional portions of these spaces may be estimated to the 

 tenths of a degree. 



Mercury has other advantages for use in thermometers. 

 It does not cling to the glass as does water and other liquids. 

 Hence its rise and fall is not interfered with by adhesive 

 force. Its boiling point of 350 C., and its freezing tempera- 

 ture of 39 C., make it serve for all ordinary temperature 

 changes. For temperatures either very high or very low 

 special forms of thermometers are employed, descriptions of 

 some of which are commonly given in Physics. Then, too, 

 the coefficient of expansion of mercury is very nearly con- 

 stant, the mercury not expanding appreciably more per 

 degree temperature change in one part of the scale than in 

 another part. 



When a confined gas at o C. has its temperature raised or 

 lowered one degree, and the pressure to which it is subjected 

 is kept the same all the time, it is found that the increase or 

 decrease in its volume is always J^ 73 of the volume at o p C. 

 If the temperature is changed 20 then the expansion or con- 

 traction is 2 %73 of that volume. " If then a gas could be 

 cooled to a temperature of 273 C., and it still remained a 

 gas, its volume theoretically would become zero. Such a tem- 

 perature is assumed to exist, and it is called absolute zero. 

 Temperatures in the centigrade scale are changed to absolute 

 readings by adding 273 to their centigrade values. 



To the scientist this possible temperature has the greatest 

 interest. Within recent years temperatures approximating ab- 

 solute zero have been attained in physical laboratories. The 

 story of how this has been done, and of the changes that take 

 place in the nature of substances subjected to these tempera- 

 tures, adds an interesting chapter to the study of Physics. 



