Specific and Latent Heat. 29 



pound's, Joule arranged a vessel containing water in such 

 a way that by means of nicely adjusted weights he could 

 cause them to drive a set of paddles in the water and by the 

 mechanical agitation warm it By knowing the number 

 of pounds in his weights, the distance they were allowed 

 to fall and the rise in temperature which was observed in 

 a given weight of water, he found the relation to be that 

 stated in (38). 



40. Specific Heat. We have learned (32) that tempera- 

 ture is a measure of the rate of molecular motion within a 

 given body ; it is not, however, a measure of the amount of 

 work which must be done upon that body to change its 

 temperature through a given number of degrees ; neither is 

 it a measure of the amount of work which may be secured 

 from that body when its temperature falls a given amount. 



When the same number of heat units is imparted to like 

 weights of different substances their temperatures are not 

 raised through an equal number of degrees. The same 

 amount of heat, for example, which will raise the tempera- 

 ture of one pound of water from 32 F. to 33 F. will 

 raise a pound of sand from 32 F. to 37.23 F. For some 

 reason more work must be done on water than on the sand 

 to secure the same change of temperature, but, true to the 

 law of the conservation of energy, when the water again 

 cools down it gives out as much more heat in doing so as 

 was required to produce the rise in temperature. It is 

 this fact which causes large bodies of water to make the 

 winters of adjacent lands warmer and the summers cooler. 

 Soils change in temperature more rapidly than would be 

 the case were their specific heats higher, and for this rea- 

 son in part a wet soil is cooler than the same soil when 

 dryer. 



41. Latent Heat. When ice at 32 F. has heat applied 

 to it its temperature does not rise so long as there is still 

 ice to melt, the whole of the energy given to it being con- 

 sumed in changing the solid ice into liquid water, that is, 



