184 



Messrs. J. B. Hannay and J. Hogarth. 



[Feb. 26. 



Alcohol . . . . 258°' 7 C 119 atmospheres. 



Carbon disulphide. 275°-0 C 77S 



The differences between Cagniard de la Tonr's nnmbers and onr 

 own in the case of alcohol are no doubt due to the difficulty of obtain- 

 ing that body anhydrous. As will be seen from the density given, the 

 alcohol used in these experiments contained only a very small quantity 

 of water, and was indeed practically anhydrous. 



To determine whether the presence of a solid in solution affected 

 the critical temperature and pressure, a cold saturated solution of 

 potassic iodide in alcohol was prepared. Experiments with this solu- 

 tion gave as the average of three trials — temperature 237°'l C, 

 pressure 71*1 atmospheres. The probable error of a single experiment 

 is for the temperature o- 37 C, for the pressure 0*5 atmosphere. The 

 exact amount of solid present in solution when the readings were made 

 it was impossible to determine ; for in altering the volume, so as to 

 observe the critical point, a small quantity was deposited on the glass 

 just at the end of the mercury column. To make sure that some salt 

 did remain in solution, we brought the mercury column up so as to 

 cover the crystalline film, and after carefully examining to see that no 

 solid particles were above the mercury, we allowed the fluid to cool 

 under pressure. When cold, the end of the capillary was broken off, 

 and the liquid, which had been shut off by the mercury from contact 

 with the crystalline deposit, received in a watch glass. On evaporation 

 it gave an appreciable residue. These experiments seem to show that 

 a liquid has its critical point raised by the solution in it of a solid. 



As the solubility of potassic iodide in alcohol is very slight (1 part 

 in 40 at 13° '5 C.) we thought it advisable to try a solution of chloride 

 of calcium in alcohol, so that the effect of solution on the pressure 

 and temperature might be shown in a more marked degree. On heat- 

 ing a cold saturated solution of calcic chloride (anhydrous), a dense 

 liquid layer was observed to form at the bottom when the temperature 

 reached 230° C. ; at 240° C. the line separating the lighter layer of 

 liquid from vapour was very indistinct, and could not be distinguished at 

 all above 250° C. ; at 255° C. the dense layer dissolved, but re-appeared 

 when pressure was taken off. "When the volume was again reduced, it 

 re-dissolved, and at 270° C. expansion caused a deposition of crystals. 

 In this experiment there was no sharp critical point, and no pressure 

 readings were taken. A dense layer, it will be remembered, was also 

 noticed in the ferric chloride experiment. A combination of the 

 chloride with part of the alcohol evidently occurs, and at a higher 

 temperature diffusion takes place until the fluid is quite homogeneous. 



While working with cobaltous chloride an interesting question pre- 

 sented itself to us — Is the absorption spectrum of a substance dissolved 

 in a fluid above its critical point the same as in liquid solution or in 



