44 Prof. Deivar, On the Colour of the Vapour [Nov. 28, 



chloride, are unequal. Hence we should expect the ferrous ion to 

 differ in intrinsic energy from the ferric ion, and to differ also in 

 properties, as we find solutions of ferrous and ferric salts to differ 

 in colour. But though we may measure the changes of energy 

 we cannot yet connect the amount of such change with any specific 

 change of properties. 



(3) On the Comparative Colour of the Vapour of Iodine in 

 Air at Atmospheric Pressure and in Vacuum. By Professor 

 Dewar. 



Recently, having had occasion to compare Iodine with other 

 vacua in the construction of the vessels used for the storage and 

 manipulation of Liquid Air in low temperature research, some 

 facts about the behaviour of the vapour of iodine have been 

 observed that deserve to be recorded. 



Pure iodine in the solid state is usually represented as being 

 perfectly opaque to light, but this is not the character of the iodine 

 distilled and condensed on a surface of glass at temperatures 

 between — 182° and - 183° C. in vacuum test-tubes or bulbs by 

 the use of "liquid air. Under such conditions it is easy to get 

 transparent films of iodine of varying grades of thickness showing 

 brilliantly the colours of thin plates by reflection; and to keep 

 them permanently as long as the low temperature is maintained. 

 The first addition of liquid air to the vacuum bulb or test-tube 

 containing excess of solid iodine causes instant precipitation of 

 an opaque film, but this can be avoided by cooling the crystals of 

 iodine which have been sublimed down to the lower part of the 

 outer test-tube or bulb by a preliminary cooling with a little solid 

 carbonic acid. In this condition when the inner surface of the 

 vacuum vessel is cooled with liquid air, the iodine can only de- 

 posit from an atmosphere of great tenuity, and, when a given 

 thickness of deposit is reached, any increase can be stopped by 

 lifting the vessel from the carbonic acid bath and placing it in 

 liquid air. In the same way films of other substances can be de- 

 posited which may be useful in the examination of many physical 

 problems. 



Stas says pure iodine gives no visible vapour at the ordinary 

 temperature. This observation of Stas is contrary to my ex- 

 perience. Samples of iodine made from subiodide of copper, from 

 iodoform, and from solution of iodine in potassium iodide, all gave 

 in half litre flasks when excess of iodine for saturation was 

 present, a visible colour to the atmosphere at the ordinary tem- 

 perature. When however a similar flask containing the same 



