300 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



M. Joannis completed the author's results, in the laboratory of 

 the latter, by a prolonged study of various simple cyanides, 

 ferrocyanides, ferricyanides, and sulphocyanides. His paper 

 will be found in full, in the " Annales de Chimie et de Physique," 

 5 e se"rie, torn. xxvi. p. 482. The author's results concerning 

 explosive substances have been given in table x., p. 132. 



2. CYANOGEN. 



1. The heat of formation of cyanogen has been measured in 

 two ways by ordinary combustion and by detonation. The 

 following is the principle upon which the calculation is based. 

 The heat of formation required depends on the heat of formation 

 of carbonic acid, which is regarded as equal to 94 Cal. for 



C (diamond) + 2 = C0 2 . 



On subtracting twice this quantity from the heat of com- 

 bustion of cyanogen, referred to the weight, which answers to the 

 equation 



C 2 N 2 + 20 2 = 2C0 2 + N* 



the difference represents the heat disengaged by the decomposi- 

 tion of the cyanogen. Consequently, this same difference taken 

 with the opposite sign, expresses the heat absorbed in the com- 

 bination of the carbon and nitrogen. 



2. It is convenient to begin with ordinary combustion, by 

 means of which the following results were obtained. 



The combustion of cyanogen by pure oxygen is easily effected 

 in the little glass combustion vessel shown on p. 241. With 

 a suitable excess of oxygen there is no formation of carbon 

 monoxide ; so that we are at once enabled to deduce the weight 

 of cyanogen consumed from the weight of carbonic acid which 

 is formed and collected in a bulbed tube (into which is subse- 

 quently introduced a lump of solid potash). 



This combustion, however, presents a complication owing 

 to the formation of a little nitric peroxide. This body is 

 absorbed by the potash, together with the carbonic acid; its 

 weight should therefore be deducted. To do this, it is deter- 

 mined by consecutive operations. For example, assuming that 

 the original nitric peroxide has, on contact with the potash, 

 been converted into nitrous and nitric acids, we can then titrate 

 the nitrous acid by means of potassium permanganate. The 

 correction resulting from this is not of much importance ; in the 

 author's experiments it varied from one to three hundredths of 

 the total weight of the carbonic acid. This correction involves 

 another of still less importance, based on the fact that the 

 formation of nitric peroxide from its elements causes an 

 absorption of heat ( 2*6) which should be added to that 



