470 PHYSIOLOGICAL PHYSICS. [Chap, xxxvn. 



small differences of temperature. For the same 

 amount of expansion the mercury will extend a 

 much longer distance in a very fine tube than 

 it would in a tube of wider calibre, so by the 

 fine tube a small difference will be readily regis- 

 tered, which would be scarcely observed in a ther- 

 mometer of wide bore. If, however, the 

 fine instrument were required to register 

 from the freezing to the boiling temperature, 

 it would require to be of a length very 

 inconvenient for practical purposes. Such 



94 



90 

 38 

 3G 



thermometers are, accordingly, made to regis- 



30 



ter only a part of the scale. Thus the 

 ordinary clinical thermometers register tem- 

 peratures between 95 and 114 Fahr., and 

 each degree is divided into fifths. An ex- 

 ample of such a thermometer is shown in 

 Fig. 195 : one which is constructed for ascer- 

 taining the internal temperature of the body. 



CHAPTER XXXVII. 



CHANGES OF STATE PRODUCED BY HEAT. 



Fusion and ebullition. Heat coii- 

 ^^^H verts solid bodies into liquids, and liquids 

 Fig. 195. into gas. Fusion is the term applied in the 

 moineter f rmer j vaporisation in the latter case. The 

 temperature at which a solid body passes 

 into the liquid state is called the point of fusion, or 

 melting point ; and it is a definite temperature for 

 each substance. The temperature at which a liquid 

 passes into vapour, which rises through its substance 

 in bubbles, and disengages itself from the surface, 



