Mr. T. Graham on the Molecular Mobility of Gases. 415 



raised in the trough. Gas gradually entered till the column of 

 mercury in the tube fell to 100 millims. The mercury was then 

 maintained at this height, by gradually raising the tube in pro- 

 portion as gas continued to enter and the mercury to fall, so as 

 to maintain a constant difference of level of 100 millims., as 

 observed by the graduation inscribed upon the tube itself, between 

 the level of the mercury in the tube and trough. The experi- 

 ment consisted in observing the time in seconds which the mer- 

 cury took to fall 10 millimetre divisions with each gas. The 

 constant volume of gas which entered was 2*2 cubic centims. 

 (01342 cubic inch). Two experiments were made with each gas. 



Oxygen entered in 898 and 894 seconds ; mean 896 seconds. 



Hydrogen in 222 and 221 seconds; mean 221'5 seconds. 



Carbonic acid in 1070 and 1060 seconds ; mean 1065 seconds. 



In such experiments the same gas exists on both sides, and 

 also occupies the pores of the diaphragm. But the molecular 

 movement within the pores in a downward direction is not fully 

 balanced by the molecular movement in an upward direction, 

 owing to the less tension, by 100 millims., of the gas below the 

 .diaphragm and within the tube than the gas above and with- 

 out. The influx of gas indicates the difference of molecular 

 movement in opposite directions. Taking the full tension of 

 the gas above the diaphragm at 760 millims., that below would 

 be 660 millims., and the movement downwards and that upwards 

 are represented by these numbers respectively. 



To increase the inequality of tension and favour the passage 

 of gas through the graphite plate, a diffusion-tube was now used, 

 48 inches in length, or of the dimensions of a barometer-tube, 

 by which a Torricellian vacuum could be commanded. The 

 pneumatic trough in which this gas-tube was suspended con- 

 sisted of a pipe of gutta percha of equal length, closed at the 

 bottom by a cork, and widening into a funnel-form at the top. 

 In one modification of the instrument it was found convenient 

 to cement a capillary glass tube to the side of the glass diffusio- 

 meter, within about 15 millims. of the upper end of the tube. 

 An opening into the upper part of the glass tube was thus ob- 

 tained, by means of which the gas contained in the diffusiometer 

 could escape when the latter was depressed in the mercurial 

 trough. A flexible tube with clip was attached to the capillary 

 tube referred to, so that the latter could be closed. From the 

 same opening a specimen of the gas contained in the diffusio- 

 meter could be drawn when required for examination. 



In another and more serviceable modification of this barome- 

 trical diffusiometer a large space was obtained above the mercu- 

 rial column, by surmounting the long glass tube, unprovided 



