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CHAPTEE III. 



HEAT OF FORMATION OF THE OXYGENATED COMPOUNDS OF NITROGEN. 



1. PRELIMINARIES. 



1. POTASSIUM nitrate, otherwise termed nitre, or saltpetre, has 

 been employed for many centuries as an ingredient of gun- 

 powder. Its use was discovered by empirical means ; but theory 

 only commenced to throw a light upon it a century ago, when 

 the part played by oxygen in combustion was discovered by 

 Lavoisier, as well as the presence of a great quantity of oxygen 

 in potassium nitrate, but the difference between these two 

 substances as regards their explosive action has only become 

 clear within the last few years, as being due not to a difference 

 in chemical composition, but rather as explicable on thermo- 

 chemical grounds. 1 



The determinations in question presented extraordinary diffi- 

 culties, and the results were not realized at the first attempt. 

 They only reached their full accuracy after a series of experiments. 



Attention has since been directed towards obtaining more 

 exact values, and the scope of the work has been extended to 

 the heat of formation of the various oxygenated compounds of 

 nitrogen, and iis theoretical importance has therefore consider- 

 ably increased. The following are the results obtained with 

 nitric oxide, which is the origin of most of the others. 



.2. HEAT OF FORMATION OF NITRIC OXIDE. 



1. The series of the five oxides of nitrogen, formed in propor- 

 tions varying according to simple ratios of weight and volume, 



1 The measurement of the heat of formation of potassium nitrate involved 

 an elaborate series of experiments, based partly on the previous determinations 

 of Dulong, Hess, Graham, Favre and Silbermann, Andrews, Wood, Thomsen, 

 Deville and Hautefeuille, Bunsen and Schischkoff, etc., but largely on experi- 

 ments begun in 1870 by the author; and the following data, relating to the 

 heat of formation of the oxygen compounds of nitrogen, were an outcome of 

 this investigation. EDS. 



