CALORIC. 



15 



The above law is universal. Whenever a solid body becomes 

 liquid, or whenever a liquid becomes gaseous, a quantity of heat dis- 

 appears, or is rendered latent; and conversely, when a gaseous 

 body is converted" into a liquid, or a liquid into a solid, a correspond- 

 ing degree of caloric is given out, or rendered free. The amount of 

 latent heat varies much in different substances. 



On this principle, the cold produced by the various frigorific 

 mixtures is explained ; thus a mixture of snow and salt produces a 

 cold of zero, in consequence of the attraction between the salt and 

 water producing liquefaction, and thereby rendering latent a large 

 amount of caloric. A notable depression of temperature is also 

 caused by a simple solution of certain salts in water, as of nitre, 

 sal ammoniac, &c. A striking example of the reverse process, or 

 the rendering latent heat sensible, is afforded in the slaking of lime 

 by water ; here, the large amount of heat evolved arises from the 

 water passing into a solid state, in its combination with the lime. 

 Latent heat has hence been denominated the heat of fluidity^ since 

 it is necessary to maintain bodies in the fluid condition. 



EFFECTS OF CALORIC. EXPANSION. 



Expansion is one of the first effects of caloric. It is caused by 

 the repelling power produced by the caloric upon the particles of the 

 body. As it is opposed to cohesion, it follows that those bodies are 

 most expanded by heat which are least influenced by cohesion ; thus 

 gases are more expansible than liquids ; and liquids more than solids. 



Expansion in Solids. — Proved, by accurate measurement before 

 and after heating; — by an accurately fitted metallic plug and ring; if 

 the plug be heated it will be too large for the ring; — the same is 

 shown in heating the tire of a wheel before hooping it ; — seen also 

 in the elongation of the metallic bar of the pyrometer, by heat. 



Of solid bodies, the metals are the most expansible. Metals are not 

 equally expansible; — lead is most so; platinum the least. If a thin, 

 straight bar of iron be firmly riveted to one of brass, and then exposed 

 to heat, the brass being more dilatable than the iron, forces the bar 

 into a curve, the convex side of which is brass; if it be artificially 

 cooled, the brass contracts more than the iron, and the reverse of the 

 above effect is produced. The supposed exception to the general 

 law that solids expand by heat in the case of clay, is only an 

 apparent one; the contraction of this substance as seen in Wedge- 

 wood's pyrometer, by the action of heat, is due to the shrinking pro- 

 duced by the loss of water. 



Expansion in Liquids. — Liquids difler' from fluids in not being 

 elastic; — they are more expansible than solids, as shown by placing 

 an alcoholic and a mercurial thermometer in the same heated sub- 



