48 CORRELATION OF PHYSICAL FORCES. 



In the first case, the quantities both of ice and water be- 

 ing indefinitely great in respect to the mercury, each reduces 

 it to its own temperature, viz. 32, and the ice cannot reduce 

 the mercury below 32, because it would receive back repul- 

 sive power from the newly formed water, and this would be- 

 come ice ; in the second case, where the quantities are limited, 

 the mercury does lose more repulsive power by the ice than by 

 the water, and the observations made in reference to the first 

 illustration apply. 



The above doctrine is beautifully instanced in the experi- 

 ment of Thilorier, by which carbonic acid is solidified. Car- 

 bonic acid gas, retained in a strong vessel under great pres- 

 sure, is allowed to escape from a small orifice ; the sudden 

 expansion requires so great a supply of force, that in furnish- 

 ing the demands of the expanding gas certain other portions 

 of the gas contract to such an extent as to solidify : thus, we 

 have reciprocal expansion and contraction going on in one 

 and the same substance, the time being too limited for the 

 whole to assume a uniform temperature,- or in other words, a 

 uniform extent of expansion. 



It has been observed with reference to heat thus viewed, 

 that it would be as correct to say, that heat is absorbed, or 

 cold produced by motion, as that heat is produced by it. This 

 difficulty ceases when the mind has been accustomed to re- 

 gard heat and cold as themselves, motion ; i. e. as correlative 

 expansions and contractions, each being evidenced by relation, 

 and being inconceivable as an abstraction. 



For instance, if the piston of an air-pump be drawn down 

 by a weight, cold is produced in the receiver. It may be here 

 said that a mechanical force, and the motion consequent upon 

 it, produces cold ; but heat is produced on the opposite side 

 of the piston, if a receiver be adapted so as to retain the com- 

 pressed air. Assuming them to equivalise each other, the 

 force of the falling weight would be expressed by the heat of 

 friction of the piston against its tube, and by the tension or 



