THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



DECEMBER 1863. 



LX. On the Molecular Mobility of Gases. 

 By Thomas Graham, F.R.S., Master of the Mint*. 



THE molecular mobility of gases will be considered at present 

 chiefly in reference to the passage of gases, under pressure, 

 through a thin porous plate or septum, and to the partial sepa- 

 ration of mixed gases which can be effected, as will be shown, by 

 such means. The investigation arose out of a renewed and 

 somewhat protracted inquiry regarding the diffusion of gases 

 (which depends upon the same molecular mobility), and has 

 afforded certain new results which may prove to be of interest in 

 a theoretical as well as in a practical point of view. 



In the Diffusiometer, as first constructed, a plain cylindrical 

 glass tube, about 10 inches in length and rather less than an 

 inch in diameter, was simply closed at one end by a porous plate 

 of plaster of Paris, about one- third of an inch in thickness, and 

 was thus converted into a gas-receiver f. A superior material 

 for the porous plate has since been found in the artificially com- 

 pressed graphite of Mr. Brockedon, of the quality used for 

 making writing-pencils. This material is sold in London in 

 small cubic masses about 2 inches square. A cube may 

 easily be cut into slices of a millimetre or two in thickness by 

 means of a saw of steel spring. By rubbing the surface of the 

 slice without wetting it upon a flat sand-stone, the thickness 

 may be further reduced to about one-half of a millimetre. A 



* From the Philosophical Transactions, Part II. for 1863, having been 

 read at the Royal Society June 18, 1863. 



t " On the Law of the Diffusion of Gases," Transactions of the Royal 

 Society of Edinburgh, vol. xii. p. 222; or Philosophical Magazine, 1834, 

 vol. ii. pp. 175, 269, 351. 



Phil. Mag. S. 4. Vol. 26. No. 177. Dec. 1863. 2 E 



