Mr. Htnry on the Materiality of Heat. 5 1 



by cohefive attraftiotl^ has tievef yet been proved : yet the 

 Various experiments of Biiffon, Whitehurtt, Fordyce, PiAetj 

 &C-. cannot be alleged as proofs that it is a6iually devoid of 

 this property > iince they only decide, that the final! quanti- 

 ties which can be artificially collected, are not to be let in 

 the balance againft the groffcr kinds of matter. One kind 

 efattraftion, 1;6i2^ which has lately been termed chemical 

 affinity, may, I think, after a full iurvey of phaenomena, be 

 fairly predfcatcd of caloric ; and if its pcMJeffion of this quality 

 be rendered probable, we fliall thence derive a powerful ar- 

 gument in favour of its materiality. 



That chemical affinity has a confiderable (liare in producing 

 the phaetlomena of heat, appears probable from the following 

 confiderations : 



1. All the charafters diftinguifhing caloric when feparate, 

 ceafe to be apparent when it has contributed to a change of 

 form in other bodies ; and the properties of the fubftances 

 fo chan2:ed are alfo materially altered-. Now this is the only 

 unequivocal mark of chemical union that we can apply in 

 any inftance ; and chemical union implies the exiftence and 

 efficiency of chemical affinity. 



2. The relation of caloric to ditTerent fubftances appears to 

 obferve that peculiar law, which, in other inftances, is termed 

 eleftive affinity. If a compound of two or more principles, 

 a metallic oxide fjr inftance, be expofcd in a high tempera- 

 ture, the caloric forms a permanent union with the one, but 

 not witli the other. In certain inftances, caloric is evolved, 

 when two fubftances, attrafting each other more powerfully 

 than they attract caloric, produce, on admixture, an elevation 

 of temperature. In other inftances, caloric is abforbed ulien 

 it is attrafted by the ne.v compound, more ftrnngly than by 

 the feparate components. Such facts warrant the dcduftion^ 

 that caloric is fubjcil to the laws of chemical aftinity. But 

 the precife order oi' its atHnities remains to be decided by fu- 

 ture experiments. 



3. Caloric Iccms alfo, on fome occafions, to bear a part 

 in llie operation of double eleftive artinities. Inthisw'ay 

 it produces dccompofitions, which, by (ingle affinitVj it is 

 incapable of eftcding. Tims a moft intenfe fire does not 

 expel entirely the carbonic acid from alkalies: but, when 

 the affinity of an acid for an alkali concurs with that of car- 

 bonic acid for caloric, a decompofition enl'nes. Agam: water 

 may be fubmilted to the higiieti temperature without impart- 

 ing a gafeous form to the hydrogen which it contains; but 

 the confpiring affinity of a metal for oxvgcn occafions the 

 produttioii of hydrogenous gas. On this piincipii- many 



^ 2, ciiemical 



