of Friction in liberating Gas from Solution. 387 



know, been explained. It is the same with a gaseous or vaporous 

 solution. Soda-water, in a chemically clean test-glass, in which 

 not a bubble of gas is visible, will display a line of bubbles along 

 the path described with friction by a clean glass, metal, or other 

 rod, against the inside of the glass below the level of the liquid. 

 So also, if a glass rod be rubbed against the side of a vessel con- 

 taining a liquid (such as spirits of wine, or a saline solution such 

 as one of common salt) at or near the boiling-point after the 

 source of heat has been removed, bubbles of vapour may be libe- 

 rated so abundantly that the liquid may be made not only to 

 boil, but to boil over. 



A gentle rubbing sometimes fails to convert a friction line into 

 a bubble line, whereas harder rubbing and a quicker motion 

 produce the effect. And in general hard bodies are more effica- 

 cious in producing the result than soft ones. It is not necessary 

 that the friction produce an actual scratching of the surface; 

 nor does the track of a bubble line remain more sensitive than 

 other parts in liberating gas after the first display is over. The 

 more highly supersaturated or superheated the solution the more 

 sensitive the surface appears to be, and the smaller the amount 

 of friction required. Beer gently warmed produces foam by 

 rubbing the side of the vessel. Or if, instead of rubbing the 

 side of the vessel, two solids such as copper and steel be intro- 

 duced into the liquid and rubbed against each other, bubbles are 

 produced. 



An explanation of these interesting facts, offered by Professor 

 Schrotter (loc. cit.) } is to the effect that the friction immediately 

 produces a change of mechanical action into latent heat or work. 



Some of my scientific friends to whom I have showed these 

 effects, endeavour to explain them by supposing that the friction 

 produces heat, or electricity, or some molecular change on the 

 surface of the vessel. 



My explanation is derived from a more vulgar source. From 

 the shelter of an archway during a heavy fall of rain I have 

 watched the extemporized puddles before me and admired the 

 large bubbles of air which frequently follow those drops which 

 plunge into the miniature lake with something like decision of 

 character. Old Mariotte was interested in the same phenomenon. 

 He says, "Each drop of rain, in falling from the height of the 

 cloud, drags with it two or three times as much air as its own 

 size, as may be shown by letting a little ball of lead fall into a 

 bucket of water ; for as soon as it touches the bottom two or 

 three bubbles of air rise, each as large as itself, which can only 

 proceed from air which follows it to the bottom of the vessel." He 

 then refers to the trompe, in which air is dragged down by water-'. 

 * CEuvres, 1/17, vol. ii. p. 353. 

 C2 



