504 PRINCIPLES OF CHEMISTRY 



to the phosphorus, and the union of the remaining elements leads to the 

 formation of hydrobrornic or hydriodic acid ; but the matter is complicated 

 by the reversibility of the reaction, the affinity for water, and other 

 circumstances which are understood by following Berthoflet's doctrine. 

 Chlorine (and bromine also) directly decomposes hydrogen suiphide, 

 forming hydrochloric acid and liberating sulphur, both in a gaseous form 

 and in solutions, whilst iodine only decomposes hydrogen sulphide in 

 weak solutions, when its affinity for hydrogen is 3 aided by the affinity of 

 hydrogen iodide for water. In a gaseous state iodine does not act 

 on hydrogen sulphide, 73 whilst sulphur is able .to decompose gaseous 

 hydriodic acid, forming hydrogen sulphide and a compound of sulphur 

 and iodine which with water forms hydriodic acid. 74 



If hydrogen sulphide be passed through water containing iodine, the 

 reaction H 2 S -f I 2 = 2HI -f S proceeds so long as the solution is 

 dilute, but when the mass of free HI Increases the reaction stops, 

 because the iodine then passes into solution. A solution having a 

 composition approximating to 2HI 4- 41 2 + 9H 2 (according to 

 Bineau) .does not react with H 2 S, notwithstanding the quantity of free 

 iodine. Therefore only weak solutions of hydriodic acid can be 

 obtained by passing hydrogen sulphide into water with iodine. 74 bi9 



To obtain 75 gaseous hydrobromic and hydriodic acids it is most 



73 This is in agreement with the thermochemical data, because if all the substances 

 be taken in the gaseous state (for sulphur the heat of fusion is 0'3, and the heat of 

 vaporisation 2'3) we have ;H 2 + S = 4:7 ; H 2 + C1 2 = 44; H 2 + Br 2 =24, and .Hjj + I^ 

 8 thousand heat units ; hence the formation of H 2 S gives less heat than that of HC1 and 

 HBr, but more than that of HI. In dilute solutions H 2 -t-6 + Aq = 9'3, and consequently 

 less than the formation of all the halogen acids, as H 2 S evolves but little heat with 

 water, and therefore in dilute solutions chlorine, bromine, and iodine decompose 

 hydrogen sulphide. 



' 4 Here there are three elements, hydrogen, sulphur, and iodine, each pair of which 

 is able to form a compound, HI, H 2 S, and SI, besides which the latter may unite in 

 various proportions. The complexity of chemical mechanics is seen in such examples as 

 these. It is evident that only the study of the simplest cases can give the key to the 

 more complex problems, and on the other hand it is evident from the examples cited in, 

 the last paged that, without penetrating into the conditions of chemical equilibria, it 

 would be impossible to explain chemical phenomena. By following the footsteps of 

 Berthollet the possibility of unravelling the problems will be reached \ but work in 

 this direction has only been begun during the last ten years, and much remains to be 

 done in collecting experimental material, for which occasions present themselves 

 at every step. In speaking of the halogens I wished to turn the reader's attention to 

 problems of this kind. 



74 bis Ti ie game essentially takes place when sulphurous anhydride, in a dilute solu- 

 tion, gives hydriodic acid and sulphuric acid with iodine. On concentration a reverse 

 reaction takes place. The equilibrated systems" and the part played by water are every- 

 where distinctly seen. 



' 5 Methods of formation and preparation are nothing more than particular cases of 

 chemical reaction. If the knowledge of chemical mechanics were more exact and com- 

 plete than it now is it would be possible to foretell all cases of preparation with every 



