1880.] On the Solubility of Solids in Gases. 181 



a high temperature was required, the experimental tube was bent 

 over as shown in the drawing, and passed into an air-bath formed of 

 two concentric iron cylinders. The cylinders had mica windows, 

 through which the experiment could be observed, and to ensure 

 perfect regularity of temperature, the inner one was lined with cement 

 about \ inch thick. As in most of our experiments it was quite un- 

 necessary to know the pressure, a simple straight tube was used in- 

 stead of the T-shaped one described. It was screwed to fit the same 

 caps, and from its simplicity made a very convenient and efficient 

 apparatus. 



The experiments with the sealed tube showed us that no deposit of 

 solid occurred above the critical point. With the assistance of the 

 apparatus just described, we were able to show a crystal of potassic 

 iodide dissolving in alcohol gas (using the word gas as Dr. Andrews 

 proposes for a fluid at any temperature above its critical point). 

 Having fixed a tube (internal diameter '75 mm., external 7 mm.), in 

 the apparatus previously filled with mercury, some alcohol was sucked 

 in by gradually withdrawing the pressure screw. By a further with- 

 drawal of the screw, the little thread of alcohol was moved inwards 

 past a constriction made in the glass tube near the end, and a small 

 fragment of fused potassic iodide dropped in so as to rest on the 

 narrow neck. Sufficient heat was now applied to boil the alcohol and 

 expel the air, and the end of the tube strongly sealed with the blow- 

 pipe. The tube being placed in its position in the bath, heat was 

 applied and the pressure regulated so as to keep the alcohol from 

 passing round the bend into the other limb of the \J tube. The tube 

 and its contents were thus raised to a temperature of 300° C. (the cri- 

 tical point of alcohol as will be shown is 234°'4 C), the crystal never 

 having come in contact with the liquid alcohol. Pressure was now 

 applied until the ateohol was reduced to about the volume occupied 

 by it while still liquid, when the fragment of iodide was seen to dissolve 

 gradually and completely. On withdrawing the screw slowly no 

 deposit occurred even when the volume was very considerably 

 increased, but by taking off the pressure very rapidly a crystalline 

 film appeared on the glass, and in some cases, a cloud of fine crystals 

 floating in the menstruum. These crystals could be easily redissolved 

 by again increasing the pressure. The deposit most frequently occurred 

 just at the surface of the mercury, and this we ascribe to a local 

 rarefaction caused by the slightly higher temperature of the mercury. 

 The fact of the existence of solvent power above the critical point 

 being established by this experiment, we next sought to examine the 

 phenomenon by using other solvents and solids. The sealed tube 

 method was found most convenient for this purpose, an experiment 

 with the pressure apparatus being resorted to when we wished to vary 

 the volume. The tube was filled from half to two-thirds with the 



