PHYSICAL FORCES 89 



Though bodies expand,* or tend to expand, through 

 the agency of heat, it is very evident that different 

 bodies and substances do nob expand at the same rate. 

 Thus it is plain that both the mercury in a thermometer 

 and the glass vessel which holds the mercury do not 

 both expand equally, or the mercury would not rise in it 

 as it does. 



Liquids expand more than solids, but different liquids 

 as well as different solids expand at different rates. 



To estimate differences of temperature it is necessary 

 to adopt a definite external standard. This is necessary 

 because not only are our feelings insufficiently persistent 

 to enable us to use them as a test, but (for reasons to be 

 explained shortly) they may positively mislead us as to 

 the relative temperatures of bodies we successively touch. 



The instrument made use of is, of course, the thermo- 

 meter, which is marked in a manner agreed upon, so 

 that it may serve as a standard of comparison. It has 

 been ascertained that the temperature at which any 

 liquid becomes solid is always the same, as also the tem- 

 perature at which any fluid boils the conditions under 

 which the ebullition takes place being similar. It is 

 this fact which enables a thermometer to be graduated 

 the freezing and boiling points of water being thus 

 constant. In England the arrangement adopted is that 

 called the scale of Fahrenheit, according to which the 

 space between the position of the mercury in the tube 

 when the thermometer is plunged into melting ice and 

 that at which it stands when in boiling water, is divided 

 into 1 80 spaces or "degrees." The space below the 

 freezing point of water is divided into 32 similar spaces ; 

 and thus, according to this system, the freezing point of 



* As to water, see post, p. 91. 



