324 



DIFFUSION OF GASES. 



volume of gas contained in the funnel ; the consequenc 

 is, that bubbles of gas escape from the end of th 

 funnel and rise through the water (fig. 207, C). If th 

 funnel, while covered with the glass plate, and in ai 

 upright position, be filled with coal gas, or still bette 



with hydrogen, and the end dipped into water, tl 

 lighter gas will diffuse outwards more rapidly thr 

 the heavier air enters inwards : the volume of gas in t ; 

 interior diminishes, and in the course of a few secon 

 the water rises to about half the height of the funn , 

 as shown in fig. 207, D. 



The mouth of the funnel should be 6 or 8 cm wide. A plate f 

 glass, somewhat larger than the mouth of the funnel, is placed h< - 

 zontally upon the table, and soft plaster of Paris is poured ove t 

 (vide p. 157), so as to form a layer 2 or at most 3 mm thick. If 

 mass does not flow well, tapping of the glass plate or strik : 

 upon the table will assist in spreading it. As soon as the laye ^ 

 sufficiently thin, the funnel is placed upon it and the edge pre. t? > 

 through the plaster, so as to cut out a disc of it, and then left in la 

 position. Half an hour after, the plaster round the funnel is remo <i 

 with a knife, and air blown through the tube ; by this means ifl 

 funnel may easily be lifted. The glass plate should be left to st u 

 in the sun or in a warm place for a few hours before an atterup i= 



