310 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



fall of temperature of - 0'510. The circulation of the gaseous 

 current is continued for twenty minutes longer, in order to 

 measure the re-heating, which, under these conditions, is very 

 slight. 



We then possess all the data necessary for calculating the 

 heat of vaporisation of hydrocyanic acid under the conditions 

 described above. 



It was found 



For HCN = 27 grms. (1st trial) 5'680 



For HCN = 27 grms. (2nd trial) . . . 5-730 



Mean ... 5-705 



Thus the formation of gaseous hydrocyanic acid from its elements, 

 according to this method 



H + C (diamond) + N = HCN" (gas), absorbs - 28*3. 



7. Solution of hydrocyanic acid. The solution of the liquid 

 acid in water may give rise to either a disengagement or to 

 an absorption of heat, according to the relative proportions, 

 and also to the temperature if the proportion of water be 

 small 1 



In this case only the heat disengaged in the presence of a 

 large quantity of water was measured. It was found that 



HCN (liquid) + 60H 2 O, at 19, disengages + 040. 



8. Third Method. Conversion of mercuric cyanide into mercuric 

 chloride, carbonic acid, and ammonia. This method consists in 

 dissolving gaseous chlorine in water contained in a closed calori- 

 meter, weighing it, and treating the solution obtained with an 

 exactly equivalent weight of mercuric cyanide ; the latter 

 becomes converted into mercuric chloride and cyanogen chloride 

 (in solution) 



i[2Cla (in solution) + Hg(CN) 2 (in solution) = HgCl 2 (in 

 solution) + 2CNC1 (in solution)]. 



The quantity of water to be employed must be calculated, so 

 as to be much greater than would be necessary to hold in 

 solution the whole of the carbonic acid finally formed. We 

 then add diluted potash, in proportions equivalent to the chlorine, 

 so as to obtain potassium chloride and potassium cyanate 



CNC1 (in solution) + K 2 (diluted) = KCNO (in solution) 

 + KC1 (diluted). 



Without troubling ourselves whether the action is more or 

 less complete, we immediately pour an excess of diluted hydro- 

 chloric acid into the same calorimeter, so as to bring the whole 



1 Bussy and Buignet, " Annales de Chimie et de Physique," 4 e se*rie, torn, 

 iii. p. 235. 



