NITROGEN TRIOXIDE. 171 



The heat of formation of nitric oxide itself from the elements 

 being 21*6, from it is deduced that of nitric peroxide gas. 

 N 4. 2 = N0 2 gas, absorbs 2'6. The formation of liquid 

 nitric peroxide on the contrary liberates heat, viz. -f 17. 



6. The figures which express the heat of formation of nitric 

 peroxide gas, whether from nitrogen and oxygen or from nitric 

 oxide and oxygen, were obtained near the ordinary temperature. 

 Their value, however, becomes notably altered when referred to 

 a higher temperature. In fact, the specific heat of nitric per- 

 oxide gas varies very considerably with the temperature. 1 This 

 gas undergoes, especially between 26 and 150, a kind of mole- 

 cular transformation of an exceptional order, which nearly 

 doubles its density, in order to bring it to a value corresponding 

 to the molecular weight N0 2 = 46 grms. This transformation 

 may be estimated by supposing the theoretical specific heat of the 

 gas constant and equal to the sum of those of its components. 



We have thus found that the transformation absorbs 



From 27 to 67 2'65 Cal. 



67 to 103 1-75 



103 to 150 0-85 



150 to 200 ... 0-03 



Total 5-28 



This number, added to the heat of vaporisation properly so 

 called, viz. 4*3, brings the heat absorbed to nearly 9 '6. Hence it 

 follows that the reaction of nitric oxide on oxygen, NO + = 

 N0 2 gas, liberates quantities of heat decreasing with tempera- 

 ture, at least from 26 up to about 200. It produces only 4- 

 13 '7 Cal. towards 200. Similarly the formation of the com- 

 pound from the elements, 



N + 2 = N0 2 , 



absorbs quantities of heat continually increasing in absolute 

 value, or 7'9 towards 200. 



These figures hardly vary from 200 to 250. Beyond this 

 they seem to decrease again, though much less rapidly, and 

 in conformity with what happens in the case of carbonic acid 

 and gases formed with condensation. 2 



7. Formation of nitrogen trioxide. The calculation of the 

 heat of formation of this acid has not been given above, because 

 it is inseparably connected with the experiments relative to 

 nitric peroxide. Take now the reaction 



2NO + = N 2 3 . 



If this reaction could take place separately, it would suffice to 

 bring together four volumes of nitric oxide and one volume of 

 oxygen, and to measure the heat liberated. 



1 " Bulletin de la Socie*te* Chimique," 2 e se*rie, torn, xxxvii. p. 434. 1882. 



2 " Essai de Me"canique Chimique," torn. i. p. 334. 



