Chemical Effects of Caloric, SfC. 79 



the conversion of a fluid into vapour. If we 

 observe the heating of water in a furnace, we 

 shall find that the heat flows into it very fast till 

 it arrives at the boiling or vaporific point. Sup- 

 pose in the last five minutes its heat is increased 

 ten degrees, in the next five we should imagine 

 it would be at least six or seven degrees more, 

 and that this would be sufficient to convert the 

 whole into vapour; and it would be so if nothing 

 more was requisite for this effect than sensible 

 heat : but this is not the case ; for in reality very 

 little of the water is evaporated, and the remain- 

 der is not sensibly hotter. 



Water more easily or with less heat boils in 

 vacuo than under the pressure of the atmosphere 

 (viz. at ninety or ninety-five degrees); but to 

 convert it all suddenly into vapour requires as 

 much, or more fuel, than in the common way, 

 over an open fire can be applied to it. 



Boyle placed some warm water, which he had 

 previously boiled, to extract its air under an ex- 

 hausted receiver. On applying heat, the water 

 boiled violently at a degree of heat not much 

 above that of the human blood. Dr, Cullen 

 also placed ether under the receiver of an air- 

 pump, into which he would let down or raise a 

 thermometer at pleasure, without admitting the 

 external air. He no sooner began to extract the 

 air than the ether was thrown into a violent agi- 

 tation or boiling, at the same time its tempera- 

 ture sunk to a surprising degree. When the 



