170 Mr. V. H. Veley on the Phases and 



water and under the influence of sunlight, the two conditions 

 required, give hypochlorous acid as one of the initial products 

 of the reaction ; the changes can therefore be written : — 



(i.) Cl 2 + 2H 2 = 2HC10 + H 2 . 



(h\) 2HC10 + 2H 3 =2HC1 + 2H 2 0. 



The above results afford a sufficient answer to the somewhat 

 sceptical remark of Ostwald : — " Ueber die Natur derZwischen- 

 Substanz Hess sich nicht bestimmtes ermitteln "*. 



As regards this phase of the reactions of hydrogen with 

 chlorine, Armstrong (Proc. Chem. Soc. 1893) writes : — " That 

 no such irregularity is observed on heating iodine with 

 hydrogen is not surprising, as hydrogen iodide would be 

 formed from the very first, and the electrolyte present would 

 exert a minimum resistance at once." But it is surprising 

 that the reactions of hydrogen with chlorine and iodine re- 

 spectively should ever be compared at all ; the only factors 

 common to the two are the element hydrogen and the some- 

 what similar behaviour of the two resulting haloid acids. 

 For the reaction between hydrogen and chlorine is conditioned, 

 as everybody now admits, by the presence of water and by 

 taking place at ordinary temperatures under the influence of 

 sunlight, and under these conditions it is probably not re- 

 versible; further it is exothermic ( + 22,000 cals.), and some 

 hydrochloric acid is formed after the lapse of about 2-4 

 minutes (Bunsen and Roscoe, Pringsheim) . On the other hand, 

 there is no experimental evidence at present to show that the 

 reaction between hydrogen and iodine is conditioned by the 

 presence of water ; it is unaffected by sunlight, as Lemoine f 

 has shown that at ordinary temperatures and after one month's 

 exposure to summer sun no appreciable combination takes 

 place, though under the same conditions 80 per cent, of 

 hydriodic acid is decomposed ; further, the reaction is re- 

 versible and endothermic ( — 800 cals.) . Lastly, it is practically 

 certain that hydriodic acid is not formed from the very first, 

 as at a temperature of 200° 97*1 per cent, of the hydrogen 

 present in a mixture of iodine jand hydrogen still remained 

 free after an interval of 9^ hours (Lemoine), wdiich, to put it 

 another way, would mean that only '005 per cent, of the 

 hydrogen has entered into combination after the lapse of 

 one minute, presuming the change to proceed uniformly. 



The phenomena of inertness and acceleration were also ob- 

 served by Berthelot and Pean de Saint Gilles in their studies 



* Lehrbuch, p. 1063. 



t Ann. Chim. Phys. [5] xii. p. 207. 



