PROFESSOR GRAHAM ON THE MOTION OF GASES. 
575 
cork this glass jet was fixed within a block-tin tube, through which the gas was to 
be drawn ; with the point of the tube directed towards the magazine of gas, so that 
the gas in passing towards the vacuum entered the conical point of the jet instead of 
issuing from it. This form of the aperture reduced the rubbing surface of glass to a 
thin ring, or made it equivalent to an aperture in a very thin plate ; but the mode of 
placing the jet, or direction in which the current passed through the aperture, was 
found afterwards to be of little consequence. 
The gas for an experiment was contained in a glass jar, of an elliptical form, 
balanced like a gasometer over water, and terminated at top and bottom with two 
short hollow cylindrical axes, of an inch in diameter ; its capacity between two marks, 
one on each of the cylindrical ends, being 22 7 cubic inches. From this gasometer 
the gas was conveyed directly into a U-shaped drying tube, 18 inches in length and 
0 - 8 inch in diameter, filled in some cases with fragments of chloride of calcium, in 
others with fragments of pumice-stone soaked in oil of vitriol ; the pumice, when 
used, having been first washed with water, to deprive it of soluble chlorides. From 
the drying tube, the gas entered the tin tube occupied by the glass jet, one end of 
that tube being connected with the drying tube, and the other with an exhausted re- 
ceiver on the plate of an air-pump. The apparatus described is exhibited in fig. 1 
of Plate XXXIII., with the exception of the elliptical gasometer, the place of which 
is occupied there by the counterpoised jar A in the water-trough. The gas was thus 
forced through the minute aperture by the whole atmospheric pressure. In making an 
experiment with any other gas than atmospheric air, a considerable quantity of the 
gas was first blown through the drying tube, from the gasometer, to displace the air 
in the former; and to do this quickly an opening was made into the air-channel 
beyond the drying tube, at G, by which gas might be allowed to escape into the 
atmosphere without proceeding further or being drawn through the glass aperture 
into the vacuum. This side aperture was closed by a brass screw and leather washer. 
In making an experiment, the gasometer was filled with the gas to be effused, and 
then connected with the air-pump receiver, in which a constant degree of exhaustion 
was maintained by continued pumping. The interval of time was noted in seconds, 
which was required for the passage of a constant volume of gas, amounting to 227 
cubic inches, namely, that contained between the two marks in the elliptical gasometer. 
Or, the volume of gas effused was more strictly 227 cubic inches, minus the volume 
of aqueous vapour which saturates air at the temperature of the experiment ; the 
vapour being withdrawn from the gas, after it left the elliptical measure and before 
it reached the effusion aperture. It is scarcely necessary to add that great care is 
necessary during these and all other experiments on gases, to maintain a uniform 
temperature. The use of a fire or stove in the room in which the experiments were 
conducted was therefore avoided, and such arrangements made that the temperature 
was kept for five or six hours within a range of a single degree of Fahrenheit’s scale. 
Hydrogen . — In the experiments first made with air and hydrogen the temperature 
