388 On the Action of Solids in liberating Gas from Solution. 



Mariotte's experiment, differently arranged, forms a good 

 illustration for a class. Over a tali glass cylinder of water is 

 suspended a funnel with its beak from 20 to 25 inches above 

 the axis of the water-jar. A shot put into the funnel will thus 

 be delivered neatly and properly to the water, and as soon as it 

 strikes the bottom a number of bubbles of air are liberated, some 

 say twenty times (Mr. Rodwell informs me thirteen times) the 

 volume of the shot*. The old idea (still retained in some mo- 

 dern books) was that the air thus liberated was air adhering to 

 the shot ; to disprove which I oiled some shot and dropped them 

 in with the same result. 



The more rational explanation is that the shot, in plunging 

 into the water, displaces a quantity of that fluid in the form of 

 a well or cylindrical shaft, to which the shot forms the lower 

 boundary, and into which air, as the more mobile body, rushes 

 before the water has time to close over it ; and as the shot is 

 still pursuing its journey, it makes a path which the cylinder of 

 air continues to follow until both are arrested at the bottom of 

 the vessel, where they are disposed of according to their respec- 

 tive densities and that of the surrounding medium. 



Now to apply this to the liberation of gas from soda-water, &c. 

 by the friction of a hard body against the side of the vessel. The 

 glass rod or the steel knitting-needle, on being pressed against 

 the side of the glass, displaces a certain small quantity of the 

 liquid, and on moving the solid, with friction, against the side 

 successive quantities of liquid are thus displaced. A certain 

 time, however short, must elapse before the water can fairly close 

 in upon the moving points of the line thus traced ; but however 

 quick the water may be in filling up the void, the gas is quicker, 

 and hence a friction line becomes a line of bubbles. 



The same explanation applies to other gaseous solutions, to 

 solutions of vapour, such as spirits of wine at or near its boil- 

 ing-point, and also to certain saline solutions. The gas, or the 

 vapour, or the salt, fills up the spaces in the friction line more 

 quickly than the liquid part of the solution can do ; and thus we 

 have a line of gas- or of vapour-bubbles, r or a line of minute 

 crystals. 



Highgate, N. October 18/4. 



• See Magnus, Pogg. Ann, vol. xcv. 



