Chap. 5-j lefs Heat than in the Air. ijt 



-Some of the phenomena of evaporation and boiling, 

 not hitherto noticed, wiJl receive a fatisfactory expla- 

 nation from this theory. ift, If a fingle drop of wa- 

 ter is heated to the vapourifk point, it is immediately 

 converted into vapour; but if the quantity is more 

 confiderable, the phenomenon will be varied: for, if a 

 quantity of water is thrown into an iron veffel heated 

 very hot, it will feem to run about the veflel like quick- 

 ill ver, but without touching the bottom or fides of the 

 veffel. The reafon is, that the water nearefl the bot- 

 tom and fides is converted into vapour, which prevents 

 the fluid from coming in contact with the iron; and 

 this is the reafon alfo that a red-hot piece of iron drop- 

 ped into water continues for fome little time in the 

 fame red-hot (late, the water neareft the iron being 

 fuddenly converted into an elaftic vapour, which repels 

 jor keeps off the reft of the fluid. 



2'dly, The bubbling and hitting of boiling fluids, or 

 of fluids upon the point of boiling, was unaccounted! 

 for till Dr. Black's theory elucidated the point. In 

 the common mode of boiling water it is plain, that the 

 bottom of the fluid arrives at the vapourific point of 

 heat before the furface; a quantity therefore of the 

 fluid which is neareft the bottom of the veffel is con- 

 verted into vapour, vi hich forcing its way through the 

 fuperincumbent medium occafions that violent ebulli- 

 tion which always takes place when a fluid is heated to 

 its vapourific point. The hiffing of kettles and other 

 veflels, previous to their arriving at the boiling point, 

 is perhaps to be accounted for from thefe vapourific 

 bubbles in their afcent meeting with the cold water, 

 and difcharging their caloric, which condenfes the va- 

 pour before it arrives at the furface, and occafions the 

 feeble found which has been juft mentioned, without 

 K 2 any 



