DIFFUSION OF GASES. 



73 



troduced 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 is filled with a paste of Paris plaster, of the usual consis- 

 tence for casts. In the course of a few minutes the plaster sets, 

 and on withdrawing the wooden cylinder, the tube forms 

 a receiver, closed by an immoveable plate of stucco. In 

 the wet state, the stucco is air-tight ; it is therefore dried, 

 either by exposure to the air for a day, or by placing it 

 in a temperature of 200 for a few hours ; and is thereafter 

 found to be permeable by gases, even in the most humid 

 atmosphere, if not positively wetted. When such a diffusion- 

 tube, six inches in length, is filled with hydrogen over mercury, 

 FIG. 2. FIG. 1. the diffusion, or exchange of air 



for hydrogen, instantly com- 

 mences through the minute pores 

 of the stucco, and proceeds with 

 so much force and velocity, that 

 within three minutes, the mer- 

 cury attains a height in the 

 receiver of more than two inches 

 above its level in the trough; 

 within twenty minutes, the whole 

 of the hydrogen has escaped. In 

 conducting such experiments over 

 water, it is necessary to avoid 

 wetting the stucco. With this 

 view, before filling the diffusion 

 tube with hydrogen, the air is 

 withdrawn by placing the tube 

 upon the short limb of an empty 

 syphon, (see Figure 1), which does 

 not reach, but comes within half an inch of the stucco, and 

 then sinking the instrument in the water trough, so that the air 

 escapes by the syphon, with the exception of a small quantity, 

 which is noted. The diffusion tube is then filled up, either 

 entirely or to a certain extent, with the gas to be diffused. 



The ascent of the water in the tube, when hydrogen is dif- 

 fused, forms a striking experiment. But in experiments made 

 with the purpose of determining the proportion between the gas 

 diffused and the air which replaces it, it is necessary to guard 

 against any inequality of pressure, by placing the diffusion tube 



