226 Mr GRAHAM on the Law of 



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 structure, 

 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 purpose, 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 mer- 

 curial trough : in forty minutes the mercury rose to a height of 

 2^ inches in the receiver above the level of the mercury in the 

 trough ; half of the hydrogen 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But these modes were superseded by the use of Paris-plaster 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 se- 

 lected, half an inch in diameter, and from six to fourteen 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 plas- 

 ter set, and, withdrawing the wooden cylinder, the tube formed 

 a receiver closed with an immoveable plug of stucco. The less 

 water employed in slaking the Paris-plaster, the more dense is 

 the plug, and the more suitable for the purpose. In the wet 

 state the plug is air-tight ; it was therefore dried, either by ex- 

 posure to the air for a day, or by placing the instrument in a 

 temperature of 200 F. for a few hours ; and thereafter was per- 



