1898.] of Iodine in Air at Atmospheric Pressure, etc. 45 



iodine was exhausted of air, the colour of the atmosphere was 

 markedly less, and this distinction remained even when the flasks 

 were heated side by side in a water bath. If, instead of air, the 

 iodine vapour diffuses into an atmosphere of carbonic acid, 

 hydrogen or oxygen, in similar flasks, placed in a water bath, the 

 colour remains to all appearance the same as in air; but in all 

 cases it is much more marked than in one from which the gaseous 

 atmosphere, other than iodine, has been in great part removed by 

 the Fluss pump. No change in the mode of operating the filling 

 of the air and vacuum flasks has made the difference in colour 

 disappear, although a dozen flasks have been filled from time to 

 time. The iodine was sublimed from anhydrous Baryta, and 

 excess of the latter was placed in some flasks along with the 

 iodine and kept for months without making any change in the 

 nature of the results. The flasks containing the Baryta, even 

 when repeatedly heated to the boiling point of water, made no 

 apparent difference in the results. This seems to prove that 

 vapour of water or hydriodic acid has nothing to do with the 

 cause of the difference in the colour of the iodine vapour in the 

 air and vacuous flasks. It is not necessary to use flasks, as two 

 lengths of glass tubing an inch or less in diameter and about 

 a foot in length, one exhausted the other not, when iieated side 

 by side in a steam or water vapour bath, show difference of 

 colour. 



To get an approximate value of the tension of the saturated 

 vapour of iodine about the ordinary temperature, a Rankine 

 formula of two terms was calculated taking the known pressure 

 at 581°C. as4 - 9mm. and that at 113 - 8°C. as 87mm. These gave: — 



2872 

 log P= 93635 y-mm (1). 



If however the tension at 85° and 1141° are selected for 

 calculation, 



31 ^7 

 log P= 10-0392-^ (2), 



where T is the absolute temperature. From this formula the 

 tensions in mm. of mercury at 0° and 11° are respectively 0'07 

 and 0*18 mm. The weight of iodine in the litre would thus 

 become about 1 and 1*94 mill, at 0° and 11° C. respectively. In 

 order to check this calculation the quantity of iodine required to 

 saturate a litre of dry air at 0° and 11° C. was determined by passing 

 a slow current of air over a column of iodine and subsequently 

 absorbing the vapour of iodine out of the saturated air current 

 in caustic potash solution. The alkaline solution on acidulation 



