HEAT 



tube which is very little altered by heat. So 

 the mercury has to expand up the tube, and 

 goes further and further as it gets hotter. This 

 gives us a measure of the temperature, for if we 

 put our thermometer into boiling water, and 

 then notice and mark how far the mercury has 

 risen in the tube, and afterwards put it into 

 freezing water, or surround it with melting ice, 

 again noticing how far back towards the bulb 

 the mercury has gone, we shall always be able 

 to tell how near the freezing point or boiling 

 point anything is in which we place our ther- 

 mometer. We can divide the space between 

 the two marks on the glass or frame as we 

 please, and make a scale, as it is called, to 

 measure the temperature. We can also find 

 out how warm it is healthy to keep our rooms 

 in winter and in summer, and what is the right 

 temperature for a hot bath. Again, by seeing 

 how far the mercury has fallen below the freezing 

 point in winter time, we can judge how severe a 

 frost there has been. In England it is never 

 so cold as to make the mercury freeze ; but in 

 an arctic winter a mercury thermometer would 

 be useless, and some other substance, which did 

 not freeze so easily, would have to be used. 



Heat, which produces the sensation of warmth 

 to a greater or less degree, according to the 

 nature of the substances through which it has to 

 travel, reaches us in three ways. It is conducted 

 to us by travelling through the substance of 



(37) 



