488 



Mr. J. B. Hannay. 



[June 10, 



are comparatively incompressible." A fluid is homogeneous above its 

 critical temperature, and it seems useless to theorise on the compressi- 

 bility of a subject not known to exist. Besides, in these statements he 

 makes temperature and pressure act in different ways. Compared 

 with gas, the liquid is more sensitive to heat, but again compared with 

 gas, the liquid is comparatively incompressible. The author says : — 

 " Carbonic anhydride, examined by Dr. Andrews, is abnormal in this 

 respect, but of this I am not certain." I must say all the bodies I 

 have examined are also " abnormal." Dr. Bamsay's idea of the critical 

 point is, that it is the temperature where their densities being equal, gas 

 and liquid mix ; but the entire absence of capillarity, showing no 

 difference in the cohesion of the upper and lower portions of the fluid, 

 and the fact that we have two portions of the same chemical com- 

 pound at the same temperature, under the same pressure with the 

 same density, would prove to any ordinary mind that they are 

 identical, as I have shown them to be in the paper already referred to. 



Dr. Bamsay seeks to explain the solubility of solids in gases by 

 saying " these gentlemen have observed nothing unusual, but merely 

 the ordinary phenomenon of the solubility of a solid in a hot liquid." 

 His former explanation would not hold here however, as the liquid 

 holding a solid in solution would have a greater density than the gas, 

 and so would not mix. Besides, suppose his explanation were true, 

 it would still prove the solubility of solids in gases, because, were the 

 solid not soluble in the gas, it would be precipitated when the liquid 

 mixed with the gas, as we know takes place when a solution is 

 mixed with a liquid in which the dissolved body is insoluble. Again, 

 according to Dr. Bamsay's theory, the higher the temperature the 

 less the solvent power would be, but the opposite is the case. He 

 attributes to me the idea that, because a solid is dissolved in a gas, it 

 is gaseous ; but I never supported that idea. Let us now consider 

 Dr. Bamsay's repetition of my experiment with potassic iodide. The 

 solubility of the iodide in alcohol is very low (1 in 40), and it increases 

 very slowly with temperature, yet, when he caused a solution saturated 

 just below the critical point to assume the gaseous condition, he 

 evidently expected a very sudden increase in the solvent power, and 

 such not being the case, he concludes that solids are not soluble in 

 gases. Then, the tube was only one-third full, and the pressure in 

 a case of that sort would be so low that the solvent power would be 

 very weak. When Br. Bamsay repeats the experiment with more 

 liquid, and raises the temperature sufficiently, he will see that the 

 solvent power is not altered in passing the critical temperature. 



An important point, which is sometimes lost sight of, is that, 

 although the pressure is independent of volume as long as the two 

 states of fluids exist together, whenever the critical temperature is 

 passed, the fluid in the vessel which contains the greatest amount 



