NA rURE 



357 



THURSDAY, AUGUST 14, 1S79 



THE DISSOCIATION OF CHLORINE 



DURING the past few years the well-known chemist, 

 Prof. V. Meyer of Zurich, has rendered signal ser- 

 vice to his brother workers by the introduction of nume- 

 rous improvements in methods of determining vapour 

 densities. At about the close of last year, in conjunction 

 with Herr C. Meyer, he described a simpler method 

 than any hitherto introduced, available for high tempera- 

 tures, and yielding results of very considerable accuracy. 



This method consisted in heating a vessel to a tem- 

 perature at which the substance whose vapour density 

 was to be determined was completely converted into gas, 

 then introducing a small weighed quantity of the sub- 

 stance in question, and subsequently measuring, at the 

 ordinary atmospheric temperature and pressure, the air 

 displaced from the vessel by the vapour of the substance. 

 In this manner, the volume of vapour, measured at the 

 atmospheric temperature and pressure, generated by a 

 known weight of substance is ascertained, and the 

 density deduced from these data by a simple calculation. 

 The great advantage of the method is that it does not 

 require a knowledge of the temperature of the vapour, 

 and the entire series of operations may be performed in a 

 very short space of time. 



The apparatus employed is also extremely simple, and 

 consists of a cylindrical bulb of about 100 c.c. capacity, 

 sealed to which is a glass tube about 6 mm. in diameter 

 and 600 mm. long ; this tube is widened out at the open 

 end, so as to admit of the introduction of a caoutchouc 

 stopper, and has a side tube, i mm. in diameter and 

 140 mm. long, sealed on to it about 100 mm. below the 

 open end. The side tube is once bent nearly at right 

 angles and the end slightly turned up, so that, when 

 dipped into water, it will deliver gas into a graduated 

 glass vessel inverted over it. For determinations at high 

 temperatures the bulb is constructed of porcelain and is 

 heated in a gas furnace ; when operating at lower tempera- 

 tures the bulb is heated either by means of a vapour bath 

 or in a bath of molten lead. The operation consists in 

 heating the bulb until it acquires a constant temperature, 

 which is indicated by the non-appearance of air-bubbles 

 at the orifice of the side tube which is plunged under 

 water ; the stopper is then removed, the weighed quantity 

 of substance introduced and allowed to fall into the bulb, 

 the stopper quickly reinserted, and the end of the side 

 tube then brought under the measuring vessel ; directly 

 air ceases to issue from the extremity of the tube, the 

 stopper is removed, and the air thus collected is after- 

 wards measured in the usual manner. In the case of 

 substances which undergo oxidation when heated in air, 

 the air is first displaced from the apparatus by a current 

 of pure nitrogen. 



Operating in this manner, the Messrs. Meyer have 

 determined the vapour density of a variety of inorganic 

 compounds, such as indium chloride, stannous chloride, 

 cuprous chloride, zinc chloride, ferric chloride, phos- 

 phorus pentasulphide, and arsenious anhydride, and have 

 obtained results showing the molecular formulae of these 

 substances to be as follows : — 



InCls, SdjCI^, CujClj, ZnCl2, FcjClj, P1S51 As40e. 

 Vot. XX. — No. 511 



They then directed their attention to elementary bodies, 

 and in a recent communication to the Berlin Chemical 

 Society, they describe the results of experiments, showing 

 that at a temperature as high as about 1,567° C. the 

 molecular composition of mercury, oxygen, nitrogen, and 

 sulphur is correctly represented by the formulas — 



Hg, O2, N2, 82, 

 which are those generally adopted. 



But with chlorine they obtained very different results. 

 They employed platinous chloride as the source of this 

 body, as it can be obtained perfectly dry and readily 

 splits up into chlorine and platinum when moderately 

 heated ; a known weight of the chloride was introduced 

 into the bulb in each experiment, the air having been 

 previously expelled by a current of nitrogen. The numbers 

 obtained in a first experiment at about 620° C. agreed 

 with those required on the assumption that the chlorine 

 molecule has the formula Clj, which is that generally 

 accepted ; but on determining the density at about 800° C, 

 a lower number was obtained, and a still lower density 

 resulted from experiments at about 1,028° C. and 1,242° C, 

 but no further change of density was observed on making 

 determinations at temperatures of about 1,392° C. and 

 1,567° C. The density (referred to air) observed at the 

 various temperatures was as follows : — 



Approximate temp. Density. 



620° 2-42 2*46 



808° 



1028° , 



1242° - 



1393° ■ 



1567° 



The density at about 1,200° and above, it will be ob- 

 served, is two-thirds of the density at 600° ; the change 

 in volume undergone by chlorine when heated is therefore 

 precisely similar to that undergone by oxygen in its 

 passage from the condition of ozone to its ordinary con- 

 dition, and it might therefore be supposed that a similar 

 change had taken place. The researches of Sir B. 

 Brodie have placed it beyond doubt that if we regard 

 ordinary oxygen as having a diatomic molecule repre- 

 sented by the formula Oy ozone has a triatomic molecule 

 of the formula O3, the conversion from ozone into oxygen 

 being represented by the equation — 



2O3 = 3O2. 

 Inasmuch, however, as chlorine has the atomic weight 

 35"4, such an explanation of the change in density of this 

 gas is inadmissible ; it would only be possible if what we 

 at present regard as the atom of chlorine is a compound 



of 3 sub-atoms each of the weight ^^-^. The only other 



3 

 explanation, however, which can be given is that chlorine 

 after all is not an element but a compound of at least two 

 elements which are dissociated by heat. Mr. Watson 

 Smith in a letter from Zurich, printed in the Chemical 

 News of last week, states that the Messrs. Meyer incline 

 to this latter explanation, and that in all probability 

 oxygen is one of the components of chlorine ; we hear also 

 from another source that they have actually separated 

 oxygen from it, but hitherto no description of this part of 

 the investigation has reached us. 



It needs not to be pointed out that such a discovery as 

 is here foreshadowed is of the highest importance. There 

 can hardly be a doubt that if chlorine be found to give way 



