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III. On the Heat of Chemical Combination, 

 By Thomas Woods, M,D, 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, Parsonstown, Dec. 13, 1852. 



IN the last Number of this Magazine Dr. Andrews disputes 

 my right to the first publication of the proof, that '' the 

 decomposition of a compound body occasions as much cold as 

 the combination of its elements originally produced heat.^' He 

 quotes a paper he published in this Magazine in 1844, and one 

 in the Philosophical Transactions for 1848, wherein he "assumes*' 

 the truth of this proposition from the fact, that when one base 

 displaces another, a quantity of heat is absorbed equivalent to 

 that produced by the combination of the former. On this 

 account he thinks he has a right by prior pubhcation to the 

 discovery. 



Now if such an assumption can be looked on in this Hght, the 

 merit of the discovery should, I believe, be given to Hess, because 

 so far back at least as 1841 he applies the knowledge of heat 

 lost by the withdrawal of a base to account for the circumstance 

 that no heat is developed in double decomposition, — this is what 

 he calls his principle of thermo -neutrality. But supposing that 

 Dr. Andrews was the first to direct attention to this point, I 

 must still be excused by him for saying, that, assuming the 

 truth of an important and constantly operating principle from 

 the agreement of one class of facts is not more than conjecture; 

 it certainly cannot be said to be a proof. I must, however, say 

 that, until Dr. Andrews directed my attention by his letter to 

 the papers he there refers to, I was not aware that he or any 

 other person had in any way alluded to the subject. If I had 

 known of the papers, I would have quoted them in mine. I 

 was only acquainted with the results of Dr. Andrews's very valu- 

 able labours. The theory I had formed of the cause of the heat 

 of chemical combination, required me to prove that the absorp- 

 tion of heat by decomposition is equal to its production by com- 

 bination, and this I attempted to do without knowing of, or 

 being able to refer to, the experiments of others, my library 

 being very limited. Through the kindness of the Earl of Eosse, 

 whose library is just the reverse of mine, I have been since then 

 enabled to supply my wants. I offer this excuse to Dr. Andrews 

 and others for not having noticed what had been said on the 

 subject previously to my paper ; but at the same time I still 

 maintain, after having made every reference to what others have 

 done, there is nothing to prevent me saying, that, by right of 

 prior pubhcation, I am the first who proved the general prin- 



