1 78 Prof. Graham on the Law of the Diffusion of Gases. 



but positive, expansion appeared to take place, the experiment 

 being performed over mercury. 



But the same fissure or opening never allows the process of 

 diffusion to go on with the same degree of rapidity in two 

 successive experiments, principally, I believe, from its size 

 changing with variations in its condition in regard to humidity. 

 The fissures appear to be extremely minute, for we cannot 

 cause either air or the gas employed to flow through them me- 

 chanically, at the same rate as it passes by the agency of dif- 

 fusion, without the application of considerable pressure. Arti- 

 ficial chinks such as that obtained by pressing together ground 

 glass-plates, or in phials fitted with accurately ground glass- 

 stoppers, allow gas to pass through under the slightest pres- 

 sure, and do not answer for the experiment. 



The effects were made much more striking, in some respects, 

 by the discovery that Wedgewood stoneware tubes, such as are 

 used in furnace experiments, admit, from their porous struc- 

 ture, of being substituted, instead of jars with fissures. When 

 shut at one end, as they are sometimes made, they may be 

 managed like other cylindrical gas-receivers. Those which 

 are unglazed are most suitable ; but do not answer the pur- 

 pose, if either very dry or too damp, being permeable by a 

 gas under the slightest pressure in the one case, and perfectly 

 air-tight in the other*. The following experiment illustrates 

 the force and rapidity with which diffusion proceeds. A 

 stoneware cylinder was entirely filled with hydrogen gas over 

 water, and transferred to the mercurial trough : in forty mi- 

 nutes the mercury rose to a height of 2~ inches in the receiver 

 above the level of the mercury in the trough ; half of the hy- 

 drogen had escaped, and had been replaced by about a third 

 of its volume of air. 



But these modes were superseded by the use of Paris- 

 plaster as the porous intermedium. 



A simple instrument, which I shall call a Diffusion-tube, 

 was constructed as follows. A glass-tube open at both ends 

 was selected, half an inch in diameter, and from six to four- 

 teen inches in length. A cylinder of wood, somewhat less in 

 diameter, was introduced into the tube, so as to occupy the 

 whole of it, with the exception of about one-fifth of an inch 

 at one extremity, which space was filled with a paste of Paris- 

 plaster of the usual consistence for castes. In the course of a 

 few minutes the plaster set, and, withdrawing the wooden cy- 



* Various facts demonstrative of the permeability to gaseous matter of 

 substances of this description, had previously been recorded by Mr. Faraday, 

 in his Bakerian Lecture on the Manufacture of Glass for Optical Purposes. 

 Phil. Trans. 1830 c.26.— Edit. 



