204* 



and double or 

 compound ; 



and .by IntcrmC' 

 diurn. 



Capacity and af. 

 finity are not 

 fynonimous 

 terms. 



On the word 

 capacity as ap- 

 plied by Craw- 

 ford. 



ON THE MATERIALITV OP CALOKIC. 



5. Caloric feems alfo, on fome occafions, to bear a part in 

 the operation of double elective affinities. In this way it pro- 

 duces decompofitions, which, by fingle affinity, it is incapable 

 of effecting. Thus a mod intenfe fire does not expel intirely 

 the carbonic acid from alkalis. But when the affinity of an 

 acid for an alkali concurs with that of carbonic acid for caloric, 

 a decompofition enfues. — Again, water may be fubmitted to 

 the higheft temperature, without imparting a gafeous form to 

 the hidrogen which it contains ; but the confpiring affinity of 

 a metal for oxigen occafions the production of hidrogenous 

 gas. On this principle, many chemical facts are refolved into 

 the law of double affinity, which are, at prefent, explained by 

 that of fingle elective attraction. 



4. Caloric acts, fometimes, as an intermedium in combin- 

 ing bodies, which, without its aid, are not fufceptible of com- 

 bination. Thus carbon and oxigen do not evince any ten- 

 dency to combination, at the ordinary temperature of the at- 

 mofphere ; but caloric brings them into union, and conftitutes, 

 itfelf, part of the refulting compound. This, and a variety 

 of other inftances, have a linking refemblance to what is call- 

 ed intermediate affinity. 



In the theory of Dr. Crawford, no influence is allowed to 

 chemical affinity over the phenomena of heat ; and indeed that 

 philofopher expreffes a decided opinion, that elementary heat 

 is not capable of uniting chemically with bodies. Hence it 

 appears, that the difference between the terms affinity and 

 capacity is not merely a verbal one ; but that they are actually 

 expreffive of different powers or Caufes : and the queflion, 

 therefore, which of thefe terms fhall be adopted, in the de- 

 fcription of fads, is one involving the determination of 

 caufes. 



The term capacity for heat is employed, by Dr. Crawford 

 and others, to denote, in the abftrad, that power, by which 

 different kinds of matter acquire different quantities of caloric. 

 But in the various applications, that are made of this theory, 

 a more precife meaning is often affixed to it ; and the term ii 

 applied, in much the fame fenfe, which it has in common 

 language. When thus understood, a difference of capacity 

 neceffarilv implies a difference in the extent of the fpaces, 

 between the minute particles of bodies ; and that thefe diffe- 

 rences occafion the varieties, obferved in the acquirement of 



hctt. 



