78 Chemistry. [Lecture 26. 



that a lump of iron dropped into water continues 

 red-hot for some little time, for the water nearest 

 the iron being instantly converted into elastic 

 vapour, keeps off the rest of the fluid. 



Water does not become hotter by being boiled 

 long. This phasnomenon has been variously ac- 

 counted for : some supposed that water will not 

 contain above a certain quantity of heat ; the ad- 

 ditional heat, they say, bursts through the fluid, 

 and gives it a bubbling motion. Others have 

 supposed that the air contained in the water be- 

 comes elastic by the heat, and forces its way 

 through the fluid. Neither of these theories is, 

 however, satisfactory ; but the true explanation 

 is easy. In the common way of boiling water, 

 the bottom of the fluid arriving at that point of 

 heat, beyond which it cannot continue without 

 being converted into vapour, is thrown up in 

 vapour to the furface, and this occasions the 

 violent ebullition observed there. Thus the 

 quantity of water is diminished, while its heat 

 is not, and cannot be, increased ; for, after the 

 fluid has been raised to its boiling point, the 

 continued application of heat converts it into 

 vapour, but does not make it hotter. In this 

 conversion of water into vapour, it will be found 

 by some experiments that the caloric applied to 

 the water disappears, and becomes latent in the 

 vapour. 



The degree of sensible heat is only necessary 

 as a condition, but is not the immediate cause of 



