. Stoichiometry . on or ex-pansion took place. The discrepancy between the twolast-named results may perhaps be ex-plained by the fact that the pressurein Thans experiment was lower thanin Devilles, and that dissociation iscomplete at the lower pressure, butnot quite at the higher. At any rate,such a discrepancy might well be ob-served at 280°, for at this temperaturethe vapour density of ammonium chloride was found by Ramsay andYoung-* to be 13-3 under a pressure of 88-4 mm., but 15-2 under apressure of 135 mm. The vapour density of tertiary amyl bromide, formed by the com-bination of amylene
. Stoichiometry . on or ex-pansion took place. The discrepancy between the twolast-named results may perhaps be ex-plained by the fact that the pressurein Thans experiment was lower thanin Devilles, and that dissociation iscomplete at the lower pressure, butnot quite at the higher. At any rate,such a discrepancy might well be ob-served at 280°, for at this temperaturethe vapour density of ammonium chloride was found by Ramsay andYoung-* to be 13-3 under a pressure of 88-4 mm., but 15-2 under apressure of 135 mm. The vapour density of tertiary amyl bromide, formed by the com-bination of amylene with hydrogen bromide, was found by Wurtzto be normal up to about 60° above its boiling point under atmos-pheric pressure ; the density then diminished with rise of tempera-ture, slowly at first, then more rapidly, and finally more slowly againuntil it became constant at half the original value. Phosphorus pentachloride has been the subject of much con-troversy and of many experiments. Cahours observed that the. Fig. 17. ^ Annalen, ly., 138 (1864). Annalen, 131, 129 (1S64).• Compt. rend., 60, 728 (1865). - Compt. rend., 56, 729 (1863).*Phil. Trans., 177, pt. i., 99 (1886).^ Ann. Chim. Phys. {3), 20, 373 (1S47). DISSOCIATION OF GASES AND VAPOURS 109 vapour density of this substance diminishes with rise of temperature,becoming constant, however, above 300°, with a value half as greatas that calculated for the formula PCI5. Deville ^ called attention to the fact that the vapour has thepeculiar colour of chlorine, and that the depth of colour increases asthe temperature rises. He pointed out that the phenomena couldbe best explained by the assumption that the pentachloride under-goes dissociation into the trichloride and chlorine. The correctness of the explanation was confirmed by Wanklynaad Robinson,- who, by the simple arrangement shown in Fig. 18,effected a partial separation of the components of phosphorus penta-chloride and also of sulphuric acid without the aid of a po
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