302 HEATS OF FOKMATION OF THE CYANOGEN SEKIES. 



The author first took, in 1871, as his starting-point 



(a) The conversion of hydrocyanic acid into formic acid and 

 ammonia. 



(Z>) The conversion of mercuric cyanide by gaseous chlorine 

 and alkalis into carbonic acid, hydrochloric acid, mercuric 

 chloride, and ammonium chloride. 



These two methods are based upon the employment of the 

 wet process. They require the knowledge of a great many 

 auxiliary data, and especially of the heat of formation of 

 ammonia. Now, the heat of formation of ammonia as adopted 

 in the first calculations, according to Thomson's measurements, 

 which were then universally accepted, was reputed to be equal 

 to + 35'15 (NH 3 in solution). As this number should be 

 reduced to + 21, according to later conclusions (p. 242), the 

 correctness of which Thomsen has himself acknowledged, it 

 became necessary to deduct the difference between these two 

 values, i.e. 14*15, from the heat of formation (from the elements) 

 of hydrocyanic acid and also from that of cyanides. But it was 

 thought necessary to check this correction by measuring the 

 heat of formation of hydrocyanic acid by means of experiments 

 of another order, which are quite independent of the heat of 

 formation of ammonia, and in which the number of auxiliary 

 data was as limited as possible. 



(c) This purpose was effected by burning a mixture of hydro- 

 cyanic acid gas and oxygen by detonation in the calorimetric 

 bomb 



2HCN + 50 = 2C0 2 + N 2 + H 2 0. 



Three data only are required in this case, viz. the heats of 

 combustion of carbon, hydrogen, and hydrocyanic acid. The 

 experiments made according to this method will be described 

 first. 



2. First Method. Combustion of hydrocyanic acid. Pure liquid 

 hydrocyanic acid is introduced, by distillation, into little phials 

 of thin glass, care being taken to keep the weight of the acid 

 within suitable limits ('14 to *1 5 of a gramme in these experi- 

 ments). These limits are regulated by the capacity of the 

 calorimetric bomb, the tension of hydrocyanic acid vapour at 

 the temperature of the experiment, and the necessity of intro- 

 ducing into the bomb a sufficient amount of oxygen to obtain 

 total combustion. The tension of hydrocyanic gas being about 

 59 of a metre at 18, i.e. almost three-quarters that of the 

 atmosphere, it is easy to fulfil the conditions required. 



The phial, sealed up and weighed, furnishes the exact weight 

 of hydrocyanic acid. This phial is carefully placed in the 

 bomb, which is then closed, and filled, by means of an orifice, 

 with pure dry oxygen at a suitable pressure. The orifice is 

 then carefully closed, and the phial containing the hydrocyanic 



