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LXXVIII. Note on the Heat of Chemical Combination. 

 By Dr. Andrews*. 



IN the last Number of this Journalj I observe that Dr. Woods, 

 referring to some observations of mine at the late meeting 

 of the British Association in this place, states that I had pre- 

 viously conjectured one of the fundamental truths suggested by 

 his theoiy, viz. "that decomposition produces as much cold 

 as the combination of the elements produced heat," and after- 

 wards claims " by right of prior publication ■" the proof of this 

 law. Dr. Woods's first paper on this subject appeared in the 

 Philosophical ]\Iagazine for October 1851. Now in a paper of 

 mine on the Thermal Changes accompanying Basic Substitutions, 

 which was published in the Philosophical Transactions for ISii, 

 and for which the Councd of the Koyal Society awarded one of 

 the Royal Medals, the following passage occurs (p. 32) : — 



" In the preceding observations it has been assumed, that if 

 the union of two substances be atterided with the evolution of a cer- 

 tain definite quantity of heat, their separation ivill be attended with 

 the absorption of the same quantity of heat. Although this pro- 

 position in the abstract is very pi'obable, it requires to be demon- 

 strated by direct experiment ; and it is the more important to 

 do so, as it will furnish, if true, a means of verifying the accu- 

 racy of our results. The reactions now described enable us to 

 test it by experiment in one particular set of cases. In fact, if 

 we take three bases, such as potash, oxide of copper, and water, 

 capable of displacing one another in the above order, and if we 

 measure the changes of temperature produced when the first and 

 second, first and third, and second and third bases displace one 

 another, then the change of temperature arising from the first 

 substitution should be equal to the difference between the changes 

 of temperature produced by the two latter. A few examples will 

 illustrate this point." 



In a subsequent paper, which appeared in the Numbers of this 

 Journal for May and June 1818, I revert to the same subject. 

 After comparing tlic quantity of heat produced by the precipita- 

 tion of metallic copper by zinc with the quantities produced by 

 the combination of those metals with oxygen, and with that due 

 to the substitution of oxide of zinc for oxide of copper, I proceed 

 to remark " that this comparison assumes the truth of the prin- 

 ciple (which I have in other inquiries endeavoured to illustrate, 

 and is indeed almost self-evident), that when, in the course of 

 any chemical reaction, the constituents of a compound are sepa- 

 rated from one another, there is a quantity of heat thereby 



* Gomniuiiicated by the Autlior. 

 Phil. May. S. l. No. 28. Supjil Vol. 4. 2 K 



