490 DR GEORGE WILSON ON THE ACTION OF THE DRY GASES 



for otherwise than by assuming the presence of water ; and the fault, moreover, 

 even if water were present, may have Iain with the coloured paper, not with the 

 gas. Tlie same objection does not apply to the following observation. If air at 

 the temperature of GO' F. be sent through a long tube filled with fused chloride 

 of calcium, it parts with moisture, Avhich the chloride absorbs and combines with. 

 If this dried air be thereafter transmitted over moist chloride of calcium, the lat- 

 ter becomes, to appearance, speedily dry. Here we have the apparently contra- 

 dictoi-y results of chloride of calcium drying air, and air drying chloride of calcium. 

 The inference seems unavoidable, that there must be a neutral point where the 

 chloride of calcium and air will be mutually indifferent, so that neither shall be 

 able to deprive the other of moisture. This point will vary in reference to, 1. the 

 relative quantities of the hygrometric salt and air acting on each other ; 2. the re- 

 lative dryness of the gas and solid ; and, 3. the temperature at which the trial is con- 

 ducted. Experiments on gases are generally made in apartments having an average 

 temperature of or about G0° F., at which the tension of water- vapour is probably 

 great enough to resist, so far as complete condensation is concerned, the absorb- 

 ing power, and affinity for it, of all hygrometrics. This remark leads directly to 

 the observation, that reduction of temperature is probably the most effectual of 

 all processes for drying a gas. It has been employed with great success by 

 Fakaday, in his later researches on the liquefaction of the gases ;* and I was in- 

 duced, in consequence, to make use of it in my experiments. The value of the 

 method admits of easy demonstration. The great obstacle to rendering a gas an- 

 hydrous, is the tension which heat confers on the water-vapour diffused through 

 it. We generally endeavour to overcome this tension by opposing to it the con- 

 densing force of porous hygrometrics, and the chemical affinity of substances 

 w^hich combine readily with water ; yet it is not at all certain that these forces 

 have the maximum condensing power attributed to them. On the other hand, it 

 is certain, that the tension of water-vapour is exceedingly small at zero, and 

 rapidly decreases as we descend the thermometric scale. 



Faraday's discovery, moreover, of the existence of a limit to vaporisation, 

 teaches that there must be a temperature at which ice abruptly ceases to give off" 

 vapour. If this point be within reach of our frigorific appliances, and were as- 

 certained, we should possess, in the reduction of gases to this temperature of no 

 vapour, a theoretically perfect process for rendering gases anhydrous. It would 

 be applicable, however, only to the less condensilile elastic fluids, for the more 

 easily liquefied ones would become liquid before the temperature of no-water va- 

 pour had been attained. It is further to be noticed, and the remark is important, 

 that all volatile bodies have their vaporising point lowered in the presence of 

 bodies more volatile than they are. The fact is familiar to every chemist. The 



* Phil. Tians., 1845. Part I., p. 155. 



