612 [June 18, 



arose out of a renewed and somewhat protracted inquiry regarding 

 the diffusion of gases (depending upon the same molecular mobi- 

 lity), 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, rather less than an inch in diameter and about ten inches in 

 length, was simply closed at one end by a porous plate of plaster of 

 paris, about one-third of an inch in thickness, and thus converted 

 into a gas receiver*. A superior material for the porous plate is 

 now found in the artificially compressed 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 circular disk 

 of this graphite, which is like a wafer in thickness but possesses con- 

 siderable tenacity, is attached by resinous cement to one end of the 

 glass tube above described, so as to close it and form a diffusiometer. 

 The tube is filled with hydrogen gas over a mercurial trough, the 

 porosity of the graphite plate being counteracted for the time by 

 covering it tightly with a thin sheet of gutta perch a. On afterwards 

 removing the latter, gaseous diffusion immediately takes place through 

 the pores of the graphite. The whole hydrogen will leave the tube 

 in forty minutes or an hour, and is replaced by a much smaller pro- 

 portion of atmospheric air (about one fourth), as is to be expected 

 from the law of the diffusion of gases. During the process, the 

 mercury will rise in the tube, if allowed, forming a column of several 

 inches in height a fact which illustrates strikingly the intensity of 

 the force with which the interpenetration of different gases is effected. 

 The native or mineral graphite is of a lamellar structure, and appears 

 to have little or no porosity. It cannot be substituted for the artifi- 

 cial graphite as a diffusion-septum. Unglazed earthenware comes 

 next in value to graphite for this purpose. 



The pores of artificial graphite appear to be really so minute, that 



* " 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 . 



