Chap, i,] Latent Heat. 91 



both of the vapour and of the remaining water im- 

 mediately iunk to the boiling point. It was evident 

 therefore that the fliperfhious heat was aoforbed by the 

 vap-ur, and as the quantity of water which was loft 

 by the procefs was not great, it followed that a confi- 

 derable quantity of the matter of heat or fire is necef- 

 fary to keep water in a (late of vapour. When any 

 quantity of heat is expelled from a body, in fuch a 

 manner as to affect our touch, it is termed, according 

 to Dr. Black's theory, Jenfible heat; and when it is 

 abforbed by any body, and exifts in combination with 

 that body, either in a fluid or vapourific (late, it is 

 termed latent heat. It is alfo evident from what has 

 been ftated, that the opinion of thefe later philofophers 

 is, that heat or fire, which has alfo been called igneous 

 fluid, matter of beat, and lately by the French che mills 

 caloric., is a diftinct fubftance or fluid, which has an at- 

 traction for all other fubftances; that it pervades mod 

 bodies ; that it is the only permanent fluid in nature, 

 and the caufe of fluidity in all other bodies. That 

 not only common fluids, fuch as water, but all elaftic 

 fluids, fuch as vapour and air, owe their exiftence in 

 that (late to the prefence of heat; and that it is fubje6t 

 to all the laws of attraction, and is more forcibly at- 

 tracted by fome bodies than by others. 



The fchool of Dr. Black feems to have confidered 

 Jight and heat as effentially different; and Dr. Scheele, 

 a Swedifli philofbpher, has endeavoured to prove, that 

 light is formed by an union of the matter of heat with 

 phlogifton or the inflammable principle : but this 

 theory is now exploded. 



Upon the theory of Dr. Black, the late ingenious 

 Pr. Crawford * has founded a very curious fyftem con- 

 cerning 



* I cannot mention this truly amiable philofopher, without a 

 fliort tribute to his memory, though it has apparently little con- 



neftioa 



