1880.] 



On the Solubility of Solids in Gases. 



489 



of liquid will develop the highest pressure on raising the temperature. 

 When, therefore, a tube contains a small amount of liquid, although 

 below the critical temperature, it develops as much pressure as a 

 tube nearly full, yet on passing that point it may not have sufficient 

 molecular closeness to exhibit solvent power, the cohesion existing 

 below the critical temperature giving it sufficient closeness there. 

 Hence, with a small quantity of liquid, the phenomenon is seen of 

 a liquid depositing a solid (which it held in solution) at the critical 

 point, but redissolving it again, when a certain increase of tempera- 

 ture gave it sufficient vis viva to compensate for its want of density. 

 With a larger volume of liquid, the solution being the same, no 

 deposit takes place. 



Then, as to the asserted difference in refractive power. I have had 

 an apparatus made specially for this purpose and, when the liquids are 

 pure, there is not the slightest difference in refractive power between 

 the upper and lower layers. Liquids which may be pure at low tem- 

 peratures may give rise to new products at higher temperatures, and 

 thus produce a different result. I have not my determinations reduced 

 yet, but will publish them shortly. They are done with a glass prism, 

 with thick sides, and the results are reliable ; but Dr. Ramsay's tube 

 may have varied more than his liquids. In another paragraph he 

 .says : — " 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." How have the two portions the same specific 

 gravity when one is always left at the bottom of the tube ? I don't 

 think any clearer proof is required to show that it is an impurity, 

 or a product of decomposition at the high temperature, and its greater 

 refraction also proves its greater density. Dr. Ramsay was very 

 careful to expel all air, and evaporated the liquid to less than half its 

 volume, thus concentrating the denser impurities in the remaining 

 portion, and as momentary exposure of a pure liquid to air slightly 

 hydrates it, we have conditions which suffice for producing the mixed 

 liquid required for the results we observed. Lastly, Dr. Ramsay 

 says : — " Surely no clearer proof is needed to show that the solids are 

 not present as gases, but simply as solution in a liquid medium," than 

 that they retain the same spectrum as in the solid state. We do 

 not want proof that they are present in the gaseous state, which is 

 very different from being dissolved in a gas. 



In another paper, I have shown that there can be no liquid above 

 the critical point, as this is the termination of all properties which 

 distinguish a liquid from a gas, so that a fluid has only one state above 

 that temperature. Solution of solids is (when chemical action does 

 not come into play) a function of all fluids, the requisite conditions 

 being molecular closeness and thermal activity. 



June 14, 1880. 



