90 Great Di/covery of Dr. Black. [Book II. 



it is kept in a fluid ftate by a quantity of fire which it 

 abforbs. . 



There is a period when the minds of men are pre- 

 pared for the reception, as well as for the profecution, 

 of great diicoveries in icience. The hints, for they are 

 little more, which had been afforded by thefe philo- 

 fophers, appear to have made little imp re (lion j and 

 the nature of heat, fire, and fluidity feems to have been 

 involved in obfcurity and contradiction, till the genius 

 and induftry of Dr. Black, of Edinburgh, developed 

 a fyftem, which explains fatisfactorily a variety of the 

 moil curious and difficult phenomena in nature. By 

 a number of nice obfervations, he was enabled to de- 

 termine that abfolute heat or fire was abforbed by all 

 bodies whatever, and that it was abforbed in greater 

 quantities by fluid than by folid fubftances; heat there- 

 fore he confidered as the caufe of fluidity. He found 

 further, that bodies in pafiing from a folid to a fluid 

 ftate abforb a quantity of heat without increafing their 

 temperature or fenfible heat, as manifefled by the ther- 

 mometer. Thus, if water with a quantity of folid ice 

 is let over the fire, the temperature of the water will 

 not be increased, but will continue at the heat of 3 2 de- 

 grees, the freezing point, tiil every particle of the ice is 

 diffolved. The reafon is, that fire or heat being ablb- 

 lutely neceflary to impart fluidity to any body, in pro- 

 portion as the ice becomes fluid the fuperfluous fire is 

 abforbed. In the fame manner, when the fluid is con- 

 verted into vapour, a quantity of abfolute heat or fire 

 is abforbed without any inert-ale of temperature above 

 the boiling or vapourific point. This difcovery Dr. 

 Black was led to by heating water in a clofe furnace a 

 confiderable degree above the boiling point ; when 

 on opening the veflel in which the water was confined, 

 he found that a fmall quantity of the fluid burft out 

 fuddenly in the form of vapour, and the temperature 



