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XVI. On the Heat developed during the Formation of the Metallic Com- 

 pounds of Chlorine, Bromine, and Iodine. By Thomas Andrews, M. D., 

 M. R. I. A., Professor of Chemistry in the Royal Belfast Institution. 



j> : y^.' • 



v Read December 12, 1842. ' . "Ir 



*li 



1. In pursuance of the train of investigation commenced in a preceding Memoir, 

 I propose, in the present communication, to advance to the consideration of the 

 more complicated thermal phenomena, which are accompanied by alterations in 

 the state of aggregation of the combining bodies. To deduce general conclusions 

 from such inquiries is extremely difficult, as the variation of temperature mea- 

 sured by the thermometer is in every instance the resultant of more than a single 

 cause, each of which must be separately eliminated, before the heat arising from 

 the chemical union can be determined. It has been my endeavour to furnish as 

 many data as possible, in the cases I have examined, for the solution of these 

 interesting problems. 



2. That we may be enabled to measure with precision the heat developed 

 during a chemical combination, it is necessary that the reaction should be very 

 quickly completed ; and the experiment is also greatly facilitated, when the ac- 

 tion commences, by simple contact, without the application of external heat. 

 These conditions are completely fulfilled, when chlorine, bromine, or iodine are 

 brought into contact with zinc or iron, water being also present. To the success 

 of the experiment the latter condition is indispensable, as these elementary bodies, 

 at ordinary temperatures, and in the dry state, have no action upon one another.* 



• The description generally given in chemical works of the rapid manner in which zinc, copper, 

 antimony, &c. enter into combination with chlorine gas at common temperatures, is only true when 

 the gas is in a moist state. Chlorine gas, when carefully dried, has no action whatever, at the ordi- 

 nary temperature of the atmosphere, upon fine filings of zinc or iron, or upon copper reduced from 

 VOL. XIX. 3 E 



