HEAT. 35 



the latter to 32 ; apply to this the doctrine of repulsive force, 

 and we get a satisfactory explanation 



In the first case, the quantity both of ice and water being 

 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 the latter would receive back re- 

 pulsive power from the newly-formed water, and this would 

 become 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 pressure, 

 is allowed to escape from a small orifice ; the sudden expan- 

 sion requires so great a supply of force, that in furnishing 

 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 

 regard 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 

 compressed air. Assuming them to be equivalent to 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 power of reaction of the compressed against the 

 dilated air. If the heat due to compression be made to perform 



D 2 



