320 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



This quantity is 19 '5 less than the heat disengaged in the direct 

 formation of mercuric chloride 



i[Hg (liquid) + C1 2 (gas) = HgCl 2 (solid)] + 314. 



4. The same quantity of heat is, on the contrary, absorbed in 

 the ordinary preparation of cyanogen, by the decomposition of 

 mercuric cyanide. To this must be added the heat of vaporisa- 

 tion of the mercury, which brings the absorption of heat to 

 about 194 for the reaction 



i[Hg(CN) 2 (solid) = Hg (gas) + (CN) 2 (gas)]. 

 We may observe that this result is very near that ( - 224) 

 which accompanies the analogous decomposition of mercuric 

 iodide into gaseous iodine and gaseous mercury. But this last 

 reaction is accompanied by phenomena of dissociation due to the 

 opposite tendency of iodine and mercury to recombine a 

 tendency which does not exist on the part of the components of 

 mercuric cyanide. 1 



5. Substitution of chlorine for the cyanogen and formation of 

 cyanogen chloride. The simple substitution of gaseous chlorine 

 for gaseous cyanogen 



i[Hg(CN) 3 (solid) + C1 2 (gas) = HgCl 2 (solid) + (ON* (gas)], 



assuming the salts to be either in the solid state or in solution 

 (the heats of solution of both salts are the same), would give off 

 .+ 194, 



In fact, this substitution is accompanied by a simultaneous 

 formation of cyanogen chloride 



^[Hg(CN) 2 (solid) + 2C1 2 = HgCl 2 (solid) + 2CNC1 (gas)] gives 



off + 21-3, 



or, if the cyanogen chloride be supposed liquid, + 2 9 '6. 



All the bodies being in solution except the chlorine, we must 

 add to this quantity the heat of solution of cyanogen chloride. 



In fact, the heat given off in this reaction, all the bodies 

 except the chlorine being in solution, was measured and found 

 to be = + 27*5 (the cyanogen chloride being also in solution). 



This figure seems to indicate that the heat of solution of 

 gaseous cyanogen chloride is very near its heat of liquefaction, 

 as might be expected. Unfortunately, this action is not instan- 

 taneous, and this fact diminishes the certainty of accuracy of the 

 estimates, and leads us to fear some complication, attributable to 

 secondary reactions of the chlorine on the water. 



6. Reciprocal displacements of the hydrochloric and hydro- 

 cyanic acids. According to the above remarks, the formation of 

 mercuric cyanide in solution, from the acid in solution and 

 precipitated mercuric oxide, gives off -f 1548, i.e. -f 6*02 more 

 than that for mercuric chloride (+ 946). The same difference 



1 " Annales de Chimie et de Physique," 5 e se*rie, torn, xviii. p. 382. 



