Prof. Graham on the Law of the Diffusion of Gases. 1 79 



Under, 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 suit- 

 able for the purpose. In the wet state the plug is air-tight; 

 if was therefore dried, either by exposure to the air for a day, 

 or by placing the instrument in a temperature of 200° Fahr. 

 for a few hours ; and thereafter was permeable by gases, even 

 in the most humid atmosphere, if not positively wetted. The 

 tube was finally graduated by means of mercury into hun- 

 dredths of a cubic inch, and the notation, as is usual with gas- 

 receivers, counted from the top. 



When such a diffusion-tube, six inches in length, was filled 

 with hydrogen over mercury, the diffusion, or exchange of air 

 for hydrogen, instantly commenced, through the minute pores 

 of the stucco, and proceeded with so much force and rapidity, 

 that within three minutes the mercury attained a height in 

 the receiver of upwards of two inches above its level in the 

 trough. Within twenty minutes the whole of the hydrogen 

 had escaped. 



In conducting such experiments over water, it was neces- 

 sary to avoid wetting the plug. With this 

 view, before filling the diffusion-tube with 

 hydrogen, the air was withdrawn by placing 

 the tube upon the short limb of an empty 

 syphon (see figure), which did not reach, but 

 came within half an inch of the plug, and 

 then sinking the instrument in the water- 

 trough, so that the air escaped by the syphon 

 with the exception of a small measure, which 

 was noted. The diffusion-tube was then 

 filled up, either entirely, or to a certain ex- 

 tent, with the gas to be diffused. 



The ascent of the water in the tube, when 

 hydrogen is diffused, forms a striking experiment. In a dif- 

 fusion-tube fourteen inches long, the water rises six or eight 

 inches in as many minutes. The column of water attains in 

 a short time its maximum height, at which, however, it is 

 never long sustained ; for as in Dcebereiner's experiment, air 

 is all along entering mechanically through the porous plug in 

 such circumstances, from the pressure of the atmosphere; and 

 after the diffusion is over, the water subsides, in the course of 

 several hours, to the general level. In experiments made with 

 the purpose of determining the proportion between the gas 

 diffused and die return-air, it was therefore necessary to guard 

 against any inequality of pressure, which was managed much 

 more easily when the tube was standing over water than over 

 mercury. 



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