68 COEEELATION OF PHYSICAL FORCES. 



a proper disposition of their constituents when expose*! to an 

 extremely low temperature, and the more so if compression 

 be also employed. 



In considering the effect of heat as a mechanical force, it 

 would be expected, a priori^ and independently of any theory 

 of heat which may be adopted, that a given amount of heat 

 acting on a given material must produce a given amount of 

 motive power ; and the next question which occurs to the 

 mind is, whether the same amount of heat would produce the 

 same amount of mechanical power, whatever be the material 

 acted on or affected by the heat. I will endeavour to reason 

 this out on the view of heat which I have advocated. Heat 

 has been considered in this essay as itself motion or mechan 

 ical power, and quantity of heat as measured by motion. 

 Thus, if by a given contraction of a body (say mercury) air 

 within a cylinder having a moveable piston be expanded, the 

 piston moves, and in this case the expansion or motion of the 

 material (say iron) of the cylinder itself and of the air sur 

 rounding it is commonly neglected. As the air dilates it be 

 comes colder ; in other words, by undergoing expansion itself, 

 it loses its power of making neighbouring bodies expand ; 

 but if the piston be forcibly kept down, the expansive power 

 due to the mercury continues to communicate itself to the 

 iron and to the surrounding air, which become hotter than 

 they would if the piston had given way. 



Now, in the above case, if the air be confined and its 

 volume unchanged, will the expansion of the iron, assuming 

 that it can be utilised, produce an exactly equivalent mechan 

 ical effect to that which the expansion of the air would pro 

 duce if the heat be entirely confined to it ? 



Assuming that (with the exception of bodies which ex 

 pand in freezing, where, through a limited range of tempera 

 ture, the converse effects obtain) whenever a body is com 

 pressed it is heated, i. e. it expands neighbouring substances ; 

 whenever it is dilated or increased in volume it is cooled, i. e 



