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



with a graphite plate, by a glass jar about half a litre in capacity. 

 This jar was more correctly a small bell-jar (fig. 4) open at top. 

 It was fitted in an inverted position, as in fig. 5, to the open end 

 of the long glass tube d, by means of a cork and cement. The 



Fig. 4. 



Fig. 5. 



Fig. 6. 



large upper opening was closed by a circular plate of gutta percha 

 (fig. 5), about 10 millims., or nearly half an inch, in thickness. 

 This disc of gutta percha had two perforations, at / and g (fig. 6), 

 the former of which was fitted above with a wide glass tube. 

 The tube /was closed below by the plate of graphite, and above 

 with a perforated cork carrying a quill tube, e. This quill tube 

 was the entrance- tube for gas, and was accompanied by the usual 

 issue-tube, i. The other aperture in the gutta-percha cover was 

 fitted with a plain quill tube, h, which did not descend below 

 the level of the gutta percha, and formed a tube of exit. No 

 difficulty was found in making all these junctions air-tight, by 

 applying the heated blade of a knife to fuse the gutta percha in 

 contact with the glass. Gutta percha is indeed of no ordinary 

 value in the construction of pneumatic apparatus. The graphite 

 plate itself required to be not less than 1 millim. in thickness, 

 in order to support the pressure of a whole atmosphere, to which 

 it is exposed in the present apparatus. This barometrical dif- 

 fusiometer is supported from above by a cord passing over a 

 pulley, and is duly counterpoised by a hanging weight. 



In operating, the first point is to expel the air from the baro- 

 meter-tube and upper chamber. The instrument (fig. 7) is sunk 

 completely in the mercurial trough previously described, till the 



