756 



HEAT. 



54. Expansion by heat. The Thermometer. An object 

 which is brought in contact with the external surface 

 of the human body generally produces sensations of 

 two kinds. Besides the sensation caused by the pressure 

 of the object against our body, we experience others 

 which induce us to characterise the object as being cold, 

 warm, hot, etc. The particular character of these sen- 

 sations is attributed by us to the particular state, in 

 relation to temperature, of the object which gives rise to 

 them, thus, if the object feels to us ' hot,' we say that 

 it has a 'high temperature ;' if it feels c cold,' we say 

 that it has a ' low temperature,' while to the common 

 cause of all these sensations, and of many other connected 

 phenomena, the name Heat is given. 



Physicists have arrived in recent times at tolerably 

 definite conclusions with reference to the nature of heat. 

 To enter befittingly upon this subject would exceed the 

 limits of this work, and it will therefore be our principal 

 aim to study the various effects of heat upon matter. 

 Of these expansion is the first to be noticed, because 

 this effect affords a means of measuring temperature. 



In fig. 379 abed represents a bent piece of brass 

 wire, so constructed that the straight bar ef exactly fits 

 into the space between the ends a and d. The piece 

 a b c d is held between c and d with the flat pliers or 



