ao8 Theories to explain the Ph^fiomena of Comhuflion. 



this water by means of iron or zinc, we rcftore it to its 

 former conibnllibility by occafioning it to imbibe light, 

 which is atforded it by the metal. If fuch decompofition is 

 etfefted bv means of iron, 73 parts of this latter unite with 

 27 of oxygen. Now, as every 37' parts o& oxvgen require 

 about four of hydrogen to form water, of courfe thefe four 

 |:^arts of hydrogen are liberated during fuch dccompofition : 

 but, as it is not probable that combuflibles are capable of 

 combining with light in all proportions, it may be afked. 

 Whether the 73 parts of iron which are oxidated contain 

 ju(l light enough, and no more, to redore the combullibiiity 

 of the four parts of hydrogen ? for, if there be too much for 

 that purpolc, the fuperabundani quantity ought to become 

 vifible; and if too little, a part only of the hydrogen {hould 

 recover its combuftibility, and be converted into gas. The 

 fame reafoning may of courfe be urged with regard to the 

 <lecompo(ition of water by any other metal ; for it is fome- 

 what fingular, that the combuftible fhould always contain 

 and give out the precife quantity of light that is fufficient to 

 reftore combuftibility to the bale of the produft of comhuf- 

 lion, and in no caie either more or lefs. Thefe, as well as 

 fome other objeftions to the above theory, were pointed out 

 by me- in a paper entitled Remarks on Dr. Thomfon's 

 Theory of Combuftion, and inferted in Nicholfon's Journal 

 for the month of July 1802. 



It fliould appear, then, that we are not in pofleffion of a fuf- 

 ficient collection of fafts to enable us to form any theory of 

 combultion that is pcrfe£fly free from objeCfion, and calcu- 

 lated to explain all the phcenomena attendant on it. Whe- 

 ther the light evolved during combuilion is emitted from thft 

 combuftible body or from the oxvgen gas, mu(i li-ill be con- 

 lidercd as a matter of doubt ; for the fuppofition lecnis per- 

 haps the more probable, that it is partly derived from both. 

 The fubtile nature of light itfelf is highly adverfe to any in- 

 quiry into its real eflence. The fame obfervation will like- 

 wife apply to caloric! The theory laft noticed is, however, 

 poirefl'ed of a high degree of merit; it fatisfaClorily explains 

 many phienomcna that \^,'ere before but little underftood ; it 

 points out, with fufficient perfpicuitv, the difference that 

 prevails between the a6l of oxysenation in bodies and that 

 of combuftion ; and though certainly liable to fome objec- 

 tions, it approximates the nearell to a fatisfaftory (txplana- 

 tion of the phenomena of combuftion of any that have hi- 

 therto been offered. 



Charles Portal. 



