DIFFUSION OF HYDROGEN. 



113 



Fig. 61. Jar of Hydro- 

 gen and lighted taper. 



Let the second jar remain at rest for twenty-four hours, and then 

 test it by thrusting up a burning taper as before. By this time the 

 hydrogen will have all diffused away and 

 become replaced by air, notwithstanding 

 that the latter is the heavier, and has to 

 rise into the jar. If the light be applied 

 before all the hydrogen has escaped, the 

 mixture of air and residual hydrogen in the 

 jar will explode with a more or less loud 

 pop, but the explosion will not be violent 

 enough to do any damage. 



Certain kinds of porous substances, more 

 especially dry plaster of Paris and unglazed 

 pottery, allow gases to diffuse through them 

 with great facility, so that gases will pass 

 much more readily under this influence than 

 when forced through by pressure. In con- 

 sequence it becomes possible to demonstrate 

 that all gases do not diffuse equally rapidly 

 under the same circumstances, but that the lighter the gas the 

 more quickly will it diffuse. 



Expt. 108. Diffusion of Hydrogen more Rapid than that of 

 Air. Make some plaster of Paris 

 into a cream with water, and place 

 it in a cup so as to fill it about a 

 quarter of an inch deep. Dip 

 into the cream a piece of glass 

 tubing about a foot long and half 

 an inch internal diameter; shortly 

 the plaster will set, so that on 

 lifting the tube out of the cup the 

 end will be blocked up by a plug 

 of plaster. Let the whole stand 

 in the air for a day or two, and 

 then bake the tube before a fire 

 or in the oven so as thoroughly to 

 dry the plaster. If the plug is 

 sound, it will now block the tube 

 so far completely that air will 

 only pass into it through the plug 



very slowly, so that by sucking 62 Diffasion of Hydrogen . 



a little air out and applying the 



tongue to the other end (taking care first to smooth the glass with 

 a file to avoid the chance of cutting the tongue), the tube will be 



