A treatise on chemistry. Vol. 1, The non-metallic elements / by Sir H.E. Roscoe & C. Schorlemmer ...

  • Henry Enfield Roscoe
Date:
1881
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on chemistry. Vol. 1, The non-metallic elements / by Sir H.E. Roscoe & C. Schorlemmer ... Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

    171/792 (page 155)
    seen by transmitted wliite light, possesses, when chemically pure, a splendid deep blue colour, but when mixed with air a reddish-violet colour (Stas). The specific gravity of iodine vapour was found by Deville and Troost to be 872 (air = 1), which corresponds to the density, 125-9, proving that the mole- cule, or two volumes of iodine-gas, weighs 126'53x2=253'06. At the ordinary temperature it volatilizes slowly, shining crystals being deposited on the sides of a bottle, on the bottom of which a little iodine has been placed. It is a bad conductor of electricity, and possesses a peculiar smell less penetrating than, though similar to that of, chlorine and bromine. The specific heat of solid iodine is, according to the experiments of Eegnault, 005412, and that of tlie liquid 0-10882. The latent heat of fluidity of iodine is ir7 thermal units, its heat of vaporization 23*95 thermal units. When an electric discharge is passed through a heated Geissler's vacuum-tube containing a trace of iodine vapour, a spectrum of bright lines is obtained, characteristic of this element.^ This emission spectrum is, however, not identical with the characteristic absorption-spectrum of iodine, so carefully mapped by Thal^n,^ and seen when white light is passed through iodine vapour. Sal^t^ has recently shown that when an electric current of feeble tension is passed through a Geissler's tube containing iodine, another set of bright bands is obtained, which are identical in position with the dark bands of Thalcjn's absorp- tion spectrum, each bright band being replaced by a black band when the vapour is illuminated from behind. 72 In its chemical properties iodine resembles chlorine and- bromine; the two latter elements have the power of displacing iodine from its combination with metals (or electro-positive elements) thus :— 2KI-hCl2=2KCl+T,.. Whilst its combinations with oxygen (or with electro-negative elements) are more stable than those of the other two elements. Thus iodine expels chlorine from the chlorates with formation of iodate and free chlorine :— 21CC10., +12 = 2KI0, + Cl^. These differences are explained when we examine the amount 1 Pliicker and Hittorf, Phil. Trans., 1865, 28. Kon Svouika Acad. Ilandb. 1869. 3 Phil. Maq. [4] 44. ].'^().