1879.] 



On the Solubility of Solids in Gases, 



o2b 



likely to yield us nrnch information. The time we have to devote to 

 scientific work being very limited, we found that it was quite im- 

 possible to make much advance by using the apparatus devised by 

 Dr. Andrews, as the time required to change from one liquid to 

 another was more than we had at our disposal. We therefore 

 devised a new apparatus, which will be described in a more lengthy 

 communication, but which, we may state, can be opened, the liquid 

 changed, and again closed for a new experiment, in about one 

 minute. 



The question as to the state of matter immediately beyond the 

 critical point being considered by Dr. Andrews to be at that time 

 incapable of receiving an answer, we imagined that some insight 

 might be gained into its condition by dissolving in the liquid some 

 solid substance whose fusing point was much above the critical point 

 of the liquid, and noticing whether, on the latter passing its critical 

 point and assuming the gaseous condition, the solid was precipitated 

 or remained in solution. We found that the solid was not deposited 

 but remained in solution, or rather in diffusion, in the atmosphere of 

 vapour, even when the temperature was raised 130° above the critical 

 point, and the gas was considerably expanded. When the side of a 

 tube containing a strong gaseous solution of a solid is approached by 

 a red hot iron, the part next the source of heat becomes coated with 

 a crystalline deposit which slowly redissolves on allowing the local 

 disturbance of temperature to disappear. Rarefaction seems to be 

 the cause of this deposition, because if the temperature be raised 

 equally and the volume retained at its original value, no deposition 

 takes place. Those experiments have been done with such solvents as 

 alcohol (ethyl ana methyl), ether, carbon disulphide and tetrachloride, 

 paraffins, and olefines, and such solids as sulphur, chlorides, bromides, 

 and iodides of the metals, and organic substances such as chlorophyll 

 and the aniline dyes. Some solutions show curious reactions at the 

 critical point. Thus ethyl alcohol, or ether, deposits ferric chloride 

 from solution just below the critical point, but re-dissolves it in the 

 gas, when it has been raised 8° or 10° above that temperature. 



It appeared to us to be of some importance to examine the spectro- 

 scopic appearances of solutions of solids when their liquid menstrua 

 were passing to the gaseous state, but as all the substances we have 

 yet been able to obtain in the two states give banded spectra with 

 nebulous edges, we are only able to state that the substance does not 

 show any appreciable change at the critical point of its solvent. Such 

 was the case with anhydrous chloride of cobalt in absolute alcohol. It 

 was suggested to us by Professor Stokes that the substance obtained 

 by the decomposition of the green colouring matter of leaves by acids, 

 and which yields a very fine absorption spectrum, might be useful for 

 our purpose. We have prepared the substance according to the care- 



