1880.] Dr. Ramsay. On the Critical State of Gases. 



327 



was filled with nearly anhydrous alcohol : and after rather more than 

 two-thirds of the alcohol had been evaporated under reduced pressure, 

 the tube was sealed. The lower portion of the tube contained a strong 

 alcoholic solution of potassium iodide, besides a small piece of undis- 

 solved salt. The upper portion of the tube was free from alcohol, 

 but its walls were encrusted with a thin crystalline film of potassium 

 iodide. The tube was heated in a sloping position, the liquid being 

 in the lower half. After the meniscus had disappeared, the iodide in 

 the lower half of the tube dissolved, while the film in the upper half, 

 even in its thinnest portions, remained unchanged. On cooling, very 

 sparkling crystals deposited in the lower half of the tube, but no 

 glittering crystals in the upper half. 



By repeated distillation the iodide in the upper portion of the tube 

 was washed down into the lower half, and when dry the sides of the 

 upper tube were quite bright and clean. The tube was again heated 

 in the same position to 20° above the temperature at which the 

 meniscus had disappeared. On cooling the sparkling crystals again 

 appeared in the lower tube, but not a trace in the upper tube. To 

 eliminate all possibility of mistake the experiment was repeated five 

 times with the same result, and finally the alcohol w r as distilled into 

 the upper tube ; it was then broken off, and its contents carefully 

 tested for iodine with sodium hypochlorite and starch-paste. There 

 was not the faintest blue colour, and it is therefore certain that 

 potassium iodide is absolutely insoluble in alcohol vapour. 



It would be interesting to speculate on the condition of a fluid, of 

 which two portions possess the same specific gravity, but refract light 

 differently. Such speculations would, however, be premature in our 

 present knowledge of the subject. 



In conclusion, I may remark, that Messrs. Hannay and Hogarth 

 found that the absorption spectrum of coloured salts remains un- 

 altered, even when the liquid in which they are dissolved loses its 

 meniscus. Surely no clearer proof is needed to show that the solids 

 are not present as gases, but are simply solutions in a liquid medium. 



(Received March 20, 1880.) 



Since the foregoing was written, Professor Stokes has been so kind 

 as to suggest in a letter to Dr. Mills, some possible causes of error, 

 which may be briefly catalogued as follows : — 



1 . The tubes may have contained air. 



That this cannot have been the case is, I think, ensured by the 

 method by which they were filled. The tube was completely filled 

 with liquid, and the liquid made to evaporate out under a vacuum of 

 25 inches till its volume had become reduced to the required amount. 

 While the reduced pressure was sustained, the tube was sealed at the 



