1825 'j Mr. Powell on Light and Heat. 403 



light and heat, it may not be irrelevant to remark, that in a late pub- 

 lication, the originality of Prof. LesHe's theory seems to be brought 

 into question. His "Inquiry" was published in 1804, and in his 

 preface he states, " that the leading facts presented themselves 

 in the spring of 1801." In the Life and Remains of Dr. Clarke 

 (4to. 491), will be found a letter from that philosopher to Mr. 

 Malthus, dated from Egypt, Sept. 9, 1801, in which he describes 

 some discussion he had had with the sgavans in that country, in 

 which he had proposed and maintained the theory that light and 

 caloric are identical, but only existing in different states. 



(5.) In reference to the history of the investigation, the fol- 

 lowing notice may also not be uninteresting. Dr. Fordyce in a 

 paper on the light produced by inflammation (Phil. Trans. 1776, 

 p. 504), makes a distinction between the light produced from 

 the inflammation or ignition of bodies, and that derived from 

 their decomposition. He proves that the latter is the case with 

 phosphorus. This light he maintains to be totally independent 

 of heat ; but there is nothing in his paper which can amount to 

 a proof of this. He considers the blue part of flame to be pro- 

 duced by decomposition, not by ignition or inflammation, which 

 is subsequently effected in the other parts of the flame. He has 

 pointed out the fact that Hght may be evolved from some sub- 

 stances, as sulphur, by the application of a less degree of heat 

 than that requisite to evolve it from the other ingredients of 

 gunpowder, though he considers this to arise from the former 

 process being not a true ignition, whilst for all real ignition one 

 particular temperature is required. 



These views must be considered curious ; and were perhaps 

 the hrst steps towards the correct theory of inflammation, since 

 so fully estabhshed by Sir H. Davy, &c. 



(6.) In the yinnal^ for May, 1824, Art. 5, p. 352, the lovers 

 of theory will find a view of the generation and natiu-e of light, 

 as deduced by Mr. Herapath from his ingenious and recondite 

 theory of evaporation, heat, &c, which exactly accords with 

 that here deduced upon principles entirely dift'erent. That phi- 

 losopher I believe in some other parts of his speculations opposes 

 the commonly received views of latent heat, and consequently 

 could not consistently bring light under the dominion of these 

 laws ; but it would seem that he regards light as in every respect 

 analogous to vapour; and thus if we admit the doctrine of 

 latent heat in the one case, the way is so far smoothed, even by 

 an opponent, for its admission in the other. 



(7.) The difficulty which I before adverted to as attaching to 

 the theory of the conversion of heat into light, viz. that only a 

 part of the heat undergoes this change, will, on the theory here 

 advanced, no longer exist. x\ny given body has only a definite 

 quantity of light m combination, and only a definite quantity of 

 heat is requisite to liberate it ; the remaming portion therefore 



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