IV. On the solubility of solids in gases. II

Abstract

In a former paper it was pointed out that a solid dissolved in a liquid was not deposited from solution when the liquid passed through the critical point into the gaseous state, and the conclusion drawn that solids are soluble in gases. I have shown in another paper that it is probable that after a liquid passes its critical point it becomes a vapour or gas, and this quite independent of pressure, and that the cohesion limit for any one substance lies more in the direction of an isothermal line than a continuation of the boiling line. Before, however, I had carried out that work, I determined to make sure that a truly gaseous state had been reached, and the fluid containing the solid was raised to 150° above its critical point, and the gaseous solution expanded to nearly twice its liquid volume, and still the solid remained in solution. In these circumstances it was found that non-volatile solids were retained in solution in truly gaseous menstrua. In the second paper referred to, I have pointed out some of the difficulties which beset such an investigation ; and when it is remembered that only air-baths can be used, the temperature being over 200° and sometimes over 300°, and that very many liquids dissociate under these temperatures, it will be seen that conclusions should be drawn only from substances perfectly free from suspicion. All the experiments, therefore, which I have performed in iron tubes cannot bear on a discussion in this matter ; and I shall defer going into the natural minerals formed artificially from aqueous solution at a red-heat till another paper. For high temperatures glass does not do, as every variety of it is easily decomposed by water ; but by the use of rock crystal blocks I have obtained some results of interest where the action of the water can be seen. However, I intend in this paper to treat in a fuller manner of work touched upon in former papers. One of the first pieces of work I set about to finish was to determine whether or not the critical points were altered by the introduction of a solid into solution. In a former paper it was thought that the critical point of alcohol containing potassic iodide in solution was higher than pure alcohol, but subsequent experiments determined me to go into the matter again. The cause was this. I found that absolute alcohol if exposed to air for a very short time takes up sufficient moisture to alter its critical point ; and I considered that in preparing the solution of the iodide some moisture might have been introduced. The experiments were repeated in the following manner. Some common absolute alcohol was placed in a flask with caustic lime, freshly burned, and cohobated for a week. The flask was then fitted with the arrangement shown in the diagram. The tube connected with the drying arrangement is turned up at the lower end, so that the exit tube may be slipped into it. A current of air desiccated over sulphuric acid is passed through this for some time, and the tube A then slipped out of the recurved end in the flask, turned round so as to clear it, and pushed down till it is below the level of the desiccated alcohol, which is forced up into the experimental tube sealed or connected to A. In this way, any quantity of alcohol, which has never been removed from contact with lime, can be introduced into the working tube. A portion of fused potassic iodide placed in the experimental tube before the experiment enables a saturated solution to be prepared out of contact with air when the liquid is sealed up. Twenty experiments were done with pure alcohol, and twenty with alcohol saturated with potassic iodide. The pure alcohol had the specific gravity .7928 at 15°.5.