114 



SCIENTIFIC AMUSEMENTS. 



pressed to the tongue by the outside atmospheric pressure, as a 

 test-tube would be if similarly treated. 



Fill the tube with hydrogen by displacement, just as the tube 

 in Expt. 76 was filled with ammonia gas, and quickly place it 

 mouth downwards in a tumbler of water ; you will soon see that 

 the water rises in the tube, so that by and by it will stand 

 at a higher level inside than it does out (fig. 62) ; showing 



that the hydrogen 

 gas has escaped out- 

 wards by diffusion 

 through the porous 

 stucco plug more 

 rapidly than air has 

 diffused inwards. 



Fig. 63 illustrates 

 a better way of 

 showing the same 

 experiment ; the 

 wider end of the 

 tapering piece of 

 glass tube is plugged 

 with stucco as be- 

 fore, and carefully 

 dried; the delivery 

 tube of a hydrogen 

 generator is passed 

 up into the diffusion 

 tube so as to dis- 

 place all air, and 

 then withdrawn ; 

 the hydrogen re- 

 servoir at the top is 

 then bent over as 

 indicated by means 

 of the connecting 

 piece of flexible 

 tubing, when diffusion takes place as before, notwithstanding that 

 the hydrogen has now to pass downwards through the plug in 

 opposition to gravity. 



Expt. 109. Another Illustration. Obtain a small circular 

 porous pot, 5 or 6 inches high and 2|- or 3 inches diameter, such 

 as is used for certain kinds of galvanic battery. Fit into the 

 mouth of the pot an india-rubber bung with a piece of glass tube 

 (a) passing tightly through it, and also passing just through a cork 



Fig. 63. Diffusion of Hydrogen. 



