JUxNE I, 1893] 



NATURE 



III 



A COMPREHENSIVE Study of the nature of the dissociation of 

 hydriodic acid gas by heat, the conditions of equilibrium of the 

 dissociated constituents, and the circumstancesunder which re- 

 combination occurs, has been made by Prof. Victor Meyer and 

 Herr ISodenstein, and their results are contributed to the current 

 number of the Berichle. The investigation was conducted upon 

 similar lines to Prof. Meyer's recent experiments upon gaseou^ 

 mixtures of hydrogen and oxygen, a series of a large number of 

 equal-sized bulbs connected by capillary tubes being simul- 

 taneously filled with the pure gas and subsequently sealed and 

 separated by fusion of the capiliaiies. In commencing the 

 experiments Prof. Meyer was surprised to observe the compara- 

 tive readiness with which gaseous iodine and hydrogen unite 

 without the aid of pladnum sponge or other condensing agents. 

 If a glass tube containing a little iodine is filled with hydrogen, 

 sealed, heated in a bath of the vapour of boiling sulphur, 

 and after cooling opened under water, a considerable escape of 

 pent-up hydriodic acid gas occurs, and the water immediately 

 afterwards ascends in the tube owing to the absorption of the 

 remainder. The hydriodic acid for the purpose of the experi- 

 ments was all prepared by the direct union of the pure elements, 

 inasmuch as the gas prepared by the usual method from iodide 

 of phosphorus was alwavs found to contain admixed volatile phos- 

 phorus compounds. The preparation was conducted by leading 

 the mixture of iodine vapour and hvdrogen over heated platinised 

 asbestos, when it was found that 86 per cent, of the iodine 

 •entered into combination. The product, after passing through a 

 suitable vessel in which the uncombined iodine was condensed, 

 was received in cooled water, the gas regenerated by warming 

 the fuming aqueous solution, and finally freed from moisture 

 by leading it over phosphoric anhydride and from the last traces 

 of free iodine by passing it over red phosphorus free from yellow 

 phosphorus and lower oxides of phosphorus. The hvdriodic 

 acid gas thus obtained proved to contain no perceptible trace of 

 impurity. Before proceeding to fill the bulbs the air was ex- 

 pelled from them by means of a current of pure hydrogen, which 

 was allowed to pass through them for 24 hours, with occasional 

 heating to near ;he softening point of the glass in order to remove 

 the film of condensed air adhering to the surface of the glass. 

 The hydrogen was finally displaced by pure hydriodic acid and 

 ■the bulbs sealed. These extreme precautions, which were adopted 

 in order to secure a number of specimens of pure hydriodic acid, 

 afford a striking example of the infinite pains which are required 

 to effect the final settlement of many of the apparently simple 

 problems of elementary chemistry. 



Prof. Meyer has definitely decided the question of the 

 action of light upon pure hydriodic acid gas. Bulbs exposed 

 upon the roof of the Heidelberg laboratory during the summer 

 months became filled in a few days with large brilliant crystals 

 of iodine. After ten days' exposure 58 per cent, of the gas had 

 been dissociated, and at the end of the summer 99 per cent., 

 or practically all. The fact that the waves of light are so 

 active in effecting dissociation rendered it imperative that the 

 thermal expeiiments should be conducted in the dark. The 

 whole of the above experiments in connection with the prepara- 

 tion of the gas and the filling of the bulbs were therefore con- 

 ducted in a dark room. The thermal results may be very 

 briefly summarised. The statement in text-books that hydriodic 

 acid commences to dissociate at 180° is incorrect. It is only 

 Ln presence of admixed air that this occurs. At 310° the 

 decomposition of the pure gas is so slight that it would take 

 2,000 hours to attain the point of maximum dissociation at 

 which equilibrium is established. This point was determined 

 indirectly to be attained when o'i669 of the original quantity 

 of gas was dissociated. At the temperature of the vapour of boil- 

 ing mercury (350') equilibrium was found directly to be estab- 

 NO. I 23 I. VOL. 48 I 



I 



lished when o'i73i was decomposed. At 394° (boiling retene) 

 o'i957 was dissociated, and at the temperature (448") of boiling 

 sulphur o'2 1 50. It is of particular interest to learn that Prof. 

 Meyer has further proved by direct experiment that the forma- 

 tion of hydriodic acid from gaseous hydrogen and iodine pro- 

 ceeds at any temperature until exactly the same condition of 

 equilibrium is attained as in the corresponding dissociation 

 experiment. Thus when the synthesis of hydriodic acid was 

 conducted at the temperature of sulphur vapour the reaction 

 proceeded until only o'2l of the elementary gases remained 

 uncombined, the same amount as was dissociated when starling 

 with the compound gas Perhaps the most interesting result 

 of the investigation is that concerning the rapidity of the dis- 

 sociation. It has been found that whenever two bulbs of equal 

 size are heated for equal lengths of time precisely the same 

 amount of decomposition or of formation occur.". The reaction 

 is thus found to proceed with strict regularity, the amount of 

 dissociation or of synthesis at any fixed temperature being a 

 direct function of the time, and capable of expression by a 

 simple mathematical formula which is given in the memoir and 

 which is amply verified by a large number of experiments. 



Notes from the Marine Biological Station, Plymouth. — Last 

 week's captures include the MoUusca Eulima distorta and 

 l\ostanga coecinea, the Isopoda Anthura gracilis and Muniia 

 Kriiyeri, and the Brachyura Eurynome aspera and Porlunus 

 marnioreus. The gelatinous alga, which has been so abundant 

 in the townettings since the beginning of April, has at length 

 almost completely disappeared. Swarms of the Leptomedusos 

 Irene pelluciJa (half-grown) and Obelia lucifera (full-grown and 

 mature) have repeatedly been taken ; but for some weeks past 

 an occasional specimen of Corymorfha nutans has been the 

 only representative of the Anthomedusje. A single large 

 Bipinnaria larva has been observed. On the shore young in- 

 dividuals of this year's growth of the Nemertines Amphiporus 

 lactifloreus and Linens obscurus (= gesserensis), and of the 

 Crustacean Carcinus mienas are now plentiful. The follow- 

 ing animals are now breeding :— Several Terebellida, the 

 Opisthobranch Philine aperia ; the Crustacea Virliitis 

 varians, Porlunus marmoreus, Slenorhynchus phalangium and 

 tenuirostris. 



The additions to the Zoological Society's Gardens during the 

 past week include a Common Hedgehog {Erinacetis europi2Us, 

 white var.) from Berkshire, presented by Mr. R. T. Hermon- 

 Hodge : a Ruddy Ichneumon {Herpesles smithi) from India, 

 presented by Mr. Maurice Welsh ; a Guillemot {Lomvia troile) 

 British, presented by Mr. H. B. Hewetson, K.Z. S. ; two Ring- 

 hals Snakes {Sepedon luvuiachetes) from South Africa, presented 

 by the Rev. G. H. R. Fisk, CM Z.S. ; an Aurora Snake 

 (Lamprophis aurora) from South Africa, presented by Mr. T. 

 E. Goodall ; a Levaillant's Amazon (Chrysalis kvaillanli) from 

 Mexico, a Grey Pariot (Psitlactts erithacus) from West Africa, 

 a Cardinal Grosbeak (Cardinalis tirginiauus), a Rose-breasted 

 Grosbeak {Hedymcles ludovicianus) from North America, 

 deposited ; a Jaguar (Felis onca, 9 ) from South America, two 

 Striped Hya;nas (Hynna slriata, S 9 ) from North Africa, a 

 Black-necked Swan (Cygnus nigricollis, i) from Antarctic 

 America, twelve Green Lizards (Lacerla viridis) South 

 European, purchased. 



OUR ASTRONOMICAL COLUMN. 



The Eclipse of April, 1893. — M. Bigourdan communi- 

 cates to Comptis Kendtts for May 23 (No. 21) a brief preliminary 

 account of his observations made during this total eclipse of 

 the sun. The station he occupied was Joal (approximaiely 

 Longitude ih. i6m. 38s. E. of Paris, and Latitude 14° 9' 25" N ) 



