EFFECTS OF HEAT. 



degrees ; those rays which they do not reflect they absorb. The 

 processes of transmission, absorption, and reflection vary with the 

 nature of the body and the state of its surface with respect to 

 smoothness, roughness, or colour. 



14. Of all the various effects of heat, that which is best adapted 

 to indicate and measure temperature is dilatation and 

 contraction. The same body always has the same 



volume at the same temperature, and always suffers 

 the same change of volume with the same change of 

 temperature. 



Since the volume and change of volume admit of 

 the most exact measurement and of the most precise 

 numerical expression, they become the means of sub- 

 mitting the degrees of warmth and cold, or, which is 

 the same, the degrees of temperature, to arithmetical 

 measure and expression. 



Although all bodies whatever are susceptible of 

 dilatation and contraction by change of temperature, 

 they are not equally convenient for thermoscopic 

 agents. 



For reasons which will become apparent hereafter, 

 the most available thermoscopic substance for general 

 purposes is mercury. 



15. The mercurial thermometer consists of a capil- 

 lary * tube of glass, (fig. 1), at one end of which a 

 thin spherical or cylindrical bulb is blown, the bulb 

 and a part of the tube being filled with mercury. 



When such an instrument is exposed to an increase 

 of temperature, the glass and mercury will both ex- 

 pand. If they expanded in the same proportion, the 

 capacity of the bulb and tube would be enlarged in 

 the same proportion as the mercury contained in 

 them, and, consequently, the column of mercury in 

 the tube would neither rise nor fall, since the enlarge- 

 ment of its volume would be exactly equal to the 

 enlargement of the capacity of the bulb and tube. If, 

 however, the expansion of the bulb and tube be differ- 

 ent from that of the mercury, the column in the tube 

 will, after expansion, stand higher or lower than 

 before, according as the expansion of the mercury is 

 greater or less than the expansion of the bulb and tube. 



It is found that the dilatability of mercury is greater than the 



* So called from its bore being so small as not to exceed the diameter of 

 a hair, the Latin word CAPILLA signifying a hair. 



149 



