EVOLUTION OF HEAT FROM WATER. 11 



lustrate another kind of power. [The Lecturer 

 here poured some water over the white sulphate 

 of copper, which - immediately became blue, 

 evolving considerable heat at the same time.] 

 Here is the same water with a substance which 

 heats nearly as much as the lime does, but see 

 how differently. So great indeed is this heat 

 in the case of lime, that it is sufficient some- 

 times (as you see here) to set wood on fire ; and 

 this explains what we have sometimes heard, 

 of barges laden with quicklime taking fire in 

 the middle of the river, in consequence of this 

 power of heat brought into play by a leakage 

 of the water into the barge. You see how 

 strangely different subjects for our considera- 

 tion arise, when we come to think over these 

 various matters the power of heat evolved by 

 acting upon lime with water, and the power 

 which water has of turning this salt of copper 

 from white to blue. 



I want you now to understand the nature of 

 the most simple exertion of this power of mat- 

 ter called weight or gravity. Bodies are heavy ; 

 you saw that in the case of water when I 

 placed i-fc in the balance. Here I have what we 

 call a weight [an iron half cwt.] a thing called 



