184 FIRE. [Lesson xxvm. 



meter. It has been called by others the caloric of 

 fluidity. 



At the moment of the chemical union of two 

 different substances, the new compound, not, per- 

 haps, having the same capacity for caloric as its 

 constituents, must either yield a part to neigh- 

 bouring bodies, or receive it from them: pro- 

 ducing thereby a change in their temperature, 

 which is increased in the former, and diminished 

 in the latter case. 



Ice, we have seen, imbibes the caloric of sur- 

 rounding bodies, until it has imbibed sufficient to 

 render it fluid j the temperature of those bodies 

 descending proportionally. On this principle may 

 be explained the effects of freezing mixtures, of 

 snow and different salts, but particularly muriate 

 of lime. During the liquefaction of these mix- 

 tures, so rapidly and so considerably is the caloric 

 absorbed, as to produce a most extraordinary de- 

 gree of cold, such as even to solidify quicksilver. 



Another change in bodies is effected by the 

 presence of heat. All liquids, and many solids,, 

 assume a gaseous form, when heated to a certain 

 temperature : thus water is made to assume the 

 form of vapour, and become 1800 times more 

 bulky than water itself. This change Dr. Black 

 discovered also to depend on a certain portion of 

 heat combining with the liquid, without producing 

 any increase of temperature. The latent heat of 

 steam was ascertained to be at least 940. The 

 Doctor proved that all liquids, during their change 

 into vapour, combine with a portion of heat, with- 

 out 



