THE HALOGENS 497 



and for HI 18000 heat units. 70 But this is especially evident from 

 the fact that solutions of hydrogen bromide and iodide in water have 

 many points in common with solutions of hydrogen chloride, both in 

 their capacity to form hydrates, fuming solutions of constant boiling 

 point, and in their capacity to form haloid salts, &c., by reacting on 

 bases. 



In consequence of what has been said above, it follows that hydro- 

 bromic and hydriodic acids, being substances which are but slightly 

 stable, cannot be evolved in a gaseous state under many of those condi- 

 tions under which hydrochloric acid is formed. Thus if sulphuric acid 

 in solution acts on sodium iodide, all the same phenomena take place as 

 with sodium chloride (a portion of the sodium iodide gives hydriodic 

 acid, and all remains in solution), but if sodium iodide be mixed with 

 strong sulphuric acid, then its oxygen decomposes the hydriodic acid 

 set free with liberation of iodine, H 2 SO 4 + 2HI = 2H,O + SO. 2 + I 2 . 

 This reaction proceeds in the reverse direction in the presence of a mass 

 of water (2000 parts of water per 1 part of SO 2 ), in which not only the 

 affinity of hydriodic acid for water evinces itself, but also the direct 

 participation of water in the direction of chemical reactions accomplished 

 through its medium. 71 Therefore, having a halogen salt, it is easy to 

 obtain gaseous hydrochloric acid (by the action of sulphuric acid), but 

 neither hydrobromic nor hydriodic acid can be so obtained free (as 

 gases). 72 Other methods are requisite for their preparation ; but above 

 all there must be conditions for the removal of oxygen, which is so easily 

 able to destroy these acids. Therefore hydrogen sulphide, phosphorus, 

 &c., which themselves easily take up oxygen, are introduced as means for 

 the conversion of bromine and iodine into hydrobromic and hydriodic 

 acids in the presence of water. For example, in the action of phosphorus 

 the essence of the matter is that the oxygen of the water goes to the 

 phosphorus, and the reaction of the remainders leads to the formation 

 of hydrobromic or hydriodic acid ; but the matter is complicated by the 

 reversibility of the reaction, the affinity for water, and other circum- 

 stances which are understood by following Berthollet's doctrine. 

 Chlorine (also bromine) directly decomposes hydrogen sulphide, forming 

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

 solutions, whilst iodine only decomposes hydrogen sulphide in weak 



70 On the basis of the data in Note 68. 



71 A number of like cases confirm what has been said in Notes 26 and 28 of 

 Chapter X. 



72 This is prevented by the reducibility of sulphuric acid. If volatile acids be taken 

 they pass over, together with the hydrobromic and hydriodic acids, when distilled ; 

 whilst many non-volatile acids which are not reduced by hydrobroraic and hydriodic 



acids only act feebly (like phosphoric acid), or do not act at all (like boric acid). 

 VOL. I. K K 



