THE EFFECTS OF HEAT. 237 



^ A thermometer does not inform us how much heat any sub- 



Does a ther- . , . , . . ^ ^, ,.„• ■ ^i 



moineter in- stance contams, but it merely pouits out the diUerence m tuo 



form us bow temperature of two or more substances. All we learn by the 

 iniich heat a ^ , ,. , j • i. 



substance con- thermometer 13 whether the temperature of one body is greater 



'^'"^ ^ or less than that of another ; and if there is a difference, it is 



expressed numerically — namely, by the degrees of the thermometer. It must 

 be remembered that these degrees are part of an arbitrary scale, selected for 

 convenience, without any reference whatever to the actual quantity of heat 

 present in bodies. 



After the ex- ^42. The first cfifect produced by heat upon 

 soudl^b heat solids is expansion. If the heat be augmented, 

 f[>ct''is°nextolf" ^^^^J changc their aggregate state and melt, 

 Kcrved? Qj. Ibecome liquid. Many solids become soft 



before melting, so that they may be kneaded ; for instance, 

 wax, glass, and iron. In this position, glass can be bent 

 and molded with facility, and iron can be forged or welded. 

 whatisLique- ^43. By Liqucfactiou we understand the 

 faction? conversion of a solid into a liquid by the 

 agency of heat, as solid ice is converted into water by the 

 heat of the sun. 



Heat is supposed to convert a soHd into a liquid, by forcing its constituent 

 particles asunder to such an extent that the force of cohesion is overcome or 

 destroyed. 



What is soiu- 544. When a solid is immersed in a liquid, 

 ''""^ and gradually disappears in it, the process 



is termed solution, and not liquefaction. A solution is 

 the result of an attraction or affinity between a solid and 

 a fluid ; and when a solid disappears in a liquid, if the 

 compound exhibits perfect transparency, we have an ex- 

 ample of a perfect solution. 



^„, . , When a fluid has dissolved as much of a solid as it is 



When IS a solu- 

 tion said to be Capable of doing, it is said to be saturated ; or, m other words, 



caturated ? ^^iQ affinity or attraction of the flui 1 for the solid continues to 



operate to a certain point, where it is overbalanced by the cohesion of tl.« 



solid; it then ceases, and the fluid is said to be saturated. 



A solution is a complete union : a mixture is a mere me- 

 llow does a , . , . ^ ,. 

 solution differ chanical union of bodies. 



from a mix- j^ most cases, the addition of heat to a liquid greatly in- 



creases its solvent properties. Hot water will dissolve much 

 more sugar than cold water ; and hot water will also dissolve many things 

 which cold water is unable to affect 



