Combination of Bodies isoith Oxt/geti and ChJorine. 433 



From a cursory inspection of the above numbers, it will be 

 evident that the quantities of heat evolved during the combi- 

 nation of different metals with chlorine or oxygen are very 

 different, varying, in the case of the compounds of chlorine, 

 from 13008 to 3114 units for each equivalent of chlorine. On 

 the other hand, there is a general resemblance between the 

 amounts of heat obtained when the same metal combines with 

 oxygen and chlorine. Thus iron yields 4134 units with oxygen, 

 4072 with chlorine; antimony, 3817 with oxygen (Dulong), 

 3804 with chlorine ; tin, 4230 with oxygen, 3966 with chlo- 

 rine. In the case of zinc there is less agreement, and in that 

 of copper, the results differ considerably ; but this may per- 

 haps arise from the compounds of chlorine with those metals 

 being obtained after each experiment in the state of aqueous 

 solutions. The determination of the heat evolved during the 

 combustion of potassium in oxygen gas would throw much 

 light on this question. The only non-metallic element ex- 

 amined is phosphorus, and it gave nearly twice as much heat 

 in combining with oxygen as with chlorine. 



It may be interesting to inquire whether the thermal effects 

 described in the foregoing extract can be connected with those 

 obtained when compounds formed of the same bodies react upon 

 one another by the moist way. Such a comparison is difficult 

 and liable to much uncertainty, from the many intermediate 

 reactions that occur during the formation of these compounds. 

 But there are two cases that admit to some extent of this 

 comparison, and it may be interesting briefly to refer to them. 



I have elsewhere shown, that when one and the same base 

 displaces another from any of its neutral combinations, the 

 same development of heat occurs* ; and in a paper lately read 

 before the Royal Society, I have endeavoured to extend a 

 similar principle to the substitutions of metals for one another, 

 and have also measured the quantities of heat evolved in many 

 reactions of this kind. For my present object, it is only ne- 

 cessary to refer to two of these results, viz. the heat due to the 

 substitution of an equivalent of oxide of zinc for oxide of cop- 

 per (353 units), and that tlue to a like substitution of metallic 

 zinc for metallic copper (3435 units). Now on the common 

 view of the constitution of salts and of their solutions, the heat 

 evolved during the precipitation of metallic copper by zinc 

 should be ecjual to the tlifference of the quantities of heat dis- 

 engaged during the combination of zinc and copper respec- 

 tively with oxygen, added to the heat i\\.\<i. to the substitution 

 of oxide of zinc lor oxide of copper. This assumes the truth 

 of the principle (which I have in other inquiries endeavoured 

 * Philosophical Transactions for 1844, p. 21. 



Fhil. Marr. S. 3. Vol. 32. No. 217. Jufie 1848. 2 F 



