THE HALOC.KNs 495 



whilst hydrogen iodide is decomposed by light at even the ordinary 

 temperature, and very easily by a red heat. And therefore the reaction 

 I L > + H. 2 = HI + HI is very easily reversible, and consequently has a 

 limit, and hydrogen iodide easily dissociates. 67 Judging by the direct 

 measurement of the heat evolved, the conversion of 2HC1 into H 2 + C1., 

 requires the expenditure of 44000 heat units (because the formation of 

 HC1 evolves 22000 heat units). The decomposition of 2HBr into 

 H. 2 + Br._, requires, if the bromine is obtained in a gaseous state, a 

 consumption of only about 24000 units, and in the decomposition 

 of 2HI into H._, + !> as vapour about 3000 heat units are evolved^ 9 * 

 which, without doubt, stands in causal connection with the great 



r>7 The (lifixocidtion of hydriodic r/r?V71ms been studied in detail by Hautefeuille and 

 Lemoine, from whose researches we will extract the following information. The decompo- 

 sition of hydriodic acid is distinct, but proceeds slowly at 180 ; the rate and limit of de- 

 composition increases with a rise of temperature The reverse action is the same that 

 is. T.-fHo forms 2HI, not only under the influence of spongy platinum (Corenwinder), 

 which also accelerates the decomposition of hydriodic acid, but it also proceeds by itself, 

 although slowly. The limit of the reverse reaction remains the same with or without 

 spongy platinum. An increase of pressure has a very powerful accelerative effect on the 

 rate of the reaction of the formation of hydriodic acid, and therefore spongy platinum l.y 

 condensing gases has the same effect as increase of pressure. At the atmospheric pressure 

 the decomposition of hydriodic acid reaches the below-mentioned limit at 250 in several 

 months, and at 440 in several hours. The limit at 250 is about 18 p.c. of decomposition 

 that is, out of 100 parts of hydrogen previously combined in hydriodic acid, about 

 10 p.c. may be disengaged at this temperature (this hydrogen may be easily measured, 

 and the measure of dissociation determined), but not more ; the limit at 440 is about 

 26 p.c. If the pressure under which 2HI passes into H., + L,be 4A atmospheres, then the 

 limit is 24 p.c. ; under a pressure of i atmosphere the limit is 29 p.c. The small influence 

 of pressure on the dissociation of hydriodic acid (compared with N.>C\>, Chap. VI. Note 

 4)'>) is due to the fact that the reaction 2HI = I 2 + H 3 is not accompanied by a change of 

 volume, and probably the existing differences are determined by deviations from Boyle- 

 Mariotte's law. In order to show the influence of time, we will cite the following figures 

 referring to 350 : 1. Reaction H.> + 1_>, 3 hours, 88 p.c. of hydrogen remained free ; 8 hours, 

 69 p.c. ; 34 hours, 48 p.c. ; 70 hours, 29 p.c.; and 327 hours, 18'5 p.c. of hydrogen remained 

 in a free state. 2. The reverse decomposition of 2HI, 9 hours, 8 p.c. of hydrogen set free, 

 and after 250 hours lsT> p.c. of hydrogen became free that is, the limit was reached. The 

 addition of extraneous hydrogen diminishes the limit of the reaction of decomposition, or 

 innvasrs the formation of hydriodic acid from iodine and hydrogen, as would be expected 

 from Berthollet's doctrine (Chap. X.). Thus at 440 26 p.c. of hydriodic acid is decom- 

 posed if there be no admixture of hydrogen, while if H., be added, then at the limit half 

 as small a mass of HI is decomposed. Therefore, if an infinite mass of hydrogen be 

 taken there will be no decomposition of the hydriodic acid. Light aids the decompo- 

 sition of hydriodic acid very powerfully. At the ordinary temperature 80 p.c. is decom- 

 posed under the influence of light, whilst under the influence of heat alone this limit 

 corresponds with a very hrjfh temperature. The distinct action of light, spongy platinum, 

 and of imperfect cleanliness in glass (especially of sodium sulphate, which destroys 

 hydriodic acid), not only render the investigations difficult, but also show that in reactions 

 like 2HI = Io + H.,, which are accompanied by slight heat effects, all foreign and feeble 

 influences may deeply affect the process of the phenomenon (Note 47). 



68 The thermal determinations of Thorn sen (at 18) gave in thousands of calories, 

 C1 + H= +22, HCl + Aq (that is, a large amount of water dissolves HC1) = + 17'3, and 



