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 lain with the coloured paper, not with the 
gas. The same objection does not apply to the following observation. If air at 
the temperature of 60° F. be sent through a long tube filled with fused chloride 
of calcium, it parts with moisture, which 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- 
dictory 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 60° 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 
Farapay, 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 
which 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 condensible 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. Trans., 1845. Part I., p. 155. 

