46 Prof. Dewar, On the Colour of the Vapour [Nov. 28, 



was titrated with hyposulphide of soda and starch solution. The 

 results of the experiments were as follows : — 



In each case the calculated tensions are less than that deduced 

 from the vapour pressure, equation (1). The values approach 

 those given by Arctowski (Zeit. Anorg. Ghem. 1895) as a deduc- 

 tion from his experiments on the volatilization of iodine. From 

 this it would follow that equation (2) for the tension is the best. 

 In the liquid state the tensions are well represented, (3) by 



log p = 7*924 ™- mm. From formula (2) the molecular latent 



heat of the solid iodine is 14430 units, and for the liquid con- 

 dition from (3) the value is 10653. The experimental value of 

 the latent heat of liquid iodine given by Favre is 6,000 units. 

 From this it would follow that the thermal values of some of the 

 physical constants of iodine require redetermination. It is inte- 

 resting to observe that the heat units required to dissociate the 

 molecule of iodine, viz. 28500, is roughly twice the calculated 

 latent heat. 



The various experiments recorded lead to the conclusion that 

 the phenomenon is a real one and not due to adventitious causes. 

 The question then remains as to its explanation. It is well known 

 that the vapour pressure in a vacuum is often greater than in air 

 at atmospheric pressure. On the other hand Professor J. J. 

 Thomson in his work entitled "Application of Dynamics to 

 Physics and Chemistry," p. 169, discusses this very question. He 

 shows that the effect of the pressure of an inert gas must be to 

 raise the vapour pressure above that given in a vacuum. Taking 

 his formula as being applicable to iodine, the difference between 

 the two conditions of pressure should amount to -^ of the whole. 

 Now the question arises, is this amount sufficient to explain 

 the difference of colour or will it be necessary to bring in other 

 factors which may operate, such as solution of solids in gas under 

 compression, or dissociation? Further experiments will be re- 

 quired of a more refined character before a definite answer to this 

 question can be given. 



Hamay and Hogarth first showed that alcohol vapour above 

 its critical point and therefore at a pressure exceeding 60 atmo- 

 spheres could dissolve solids like bromide and iodide of potassium ; 

 and Cailletet a little later showed that liquid carbonic acid was 

 dissolved by air under high compression. Dr Villard has recently 



