io6 On the different Theories of Fhllofophefs 



effcvS may be prodaced without any of the phaeiiomena ufa- 

 ally attendant on coinbuiiion; and that, though certainly all 

 conibndion prefuppofes the combination of oxygen with a 

 bafc, yet this conibination may be_, and repeatedly is, etfe^led 

 where-no combuftion can poffibly take place. 



It is the objeiSt, therefore, of Dr. 'I homfon's theory to 

 point out the difference which in nuniberlels inftances pre- 

 vails between the aft of oxygenation of bodies and that of 

 comburtinn, and particularly to account, in a more fatistac- 

 tory manner than lias hitherto been done, for the emiflion 

 that takes place during combuftion of light and caloric. 'J'hc 

 two following are the leading politions of the doftor's theory : 



all, That during comhuliion all combunibles eujit light, 

 which previoi;(!v formed a neceflary ingredient to their own 

 compofition : and, 



2dly, 'Ih^t the heat which is evolved during the procefs of 

 combuftion proceeds from the decompofition of oxygen gas. 



It has been before obrerved, that by the phlogiftic theory 

 the light and beat are fuppofed to proceed frcmi the combuf- 

 tible body ; but that by the theory of Lavoifier they are held 

 to proceed from the deeompoftlion of oxygen gas, of which 

 body thev are confidercd as forming conftituent parts. 



In the iniancy of chemical knowledge, and before the dif- 

 eovery of vital air, that the extrication of heat proceeded from 

 the combufiible body, was the only natural eonclufion that 

 could preient itfelf to the mind ; and as light and hqat were 

 confidercd as only modifications of the fame fubftance, the 

 fu])porition of courfe prevailed that both were evolved from 

 the inflammable body. < 



Since, however, the later experiments of phllofophers, and 

 particularly thofe lA Drs. Herlchel and Woolafton, have dif- 

 proved this fuppolcd iJentity, it evidently becomes no longer 

 iieceffery to trace thele two fubftances to the fame fourc^; 

 and this, as is before ftatcd, is the cafe with the theory we 

 are now reviewing. 



That the caloric which is evolved during combuftion fliould 

 proceed from the decompofition of the oxygen gas, is, amongft 

 a variety of other rcafuns, rendered the more probable from 

 the confidcration that bodies poflefs a greater fliare of caloric 

 in thegaleous ftate than in any other; and confetpiently the 

 probability is greatly ftrengthened, that the heat which is 

 evolved during the procefs of combuftion proceeds rather 

 from the oxygen gas than from the inflammable matter, and 

 that it is from the condenfation of oxygen gas that caloric is 

 chiefly evolved ; or, according to the explanation of Lavoi- 

 fier, that the oxygen of the gas, poficfling a ftronsrer afl:inity 



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