connected with the Boiling of Liquids* 447 



nishes and disappears. For example, in distilling wood-spirit 

 (boiling-point 148° F.), a small tube, held by a thin wire passing 

 through the cork, stood with its mouth near the bottom of the 

 retort. After the fifth boiling, when the lamp had been removed, 

 the tube poured out about 150 bubbles of vapour before it began 

 to fill. It occupied three minutes in filling, and then a minute 

 speck was visible by the aid of a lens ; but within another minute 

 the liquid closed over this speck and it disappeared, the tempe- 

 rature of the liquid being as high as 128° F. M. Gernez would 

 say that the speck had entered into solution, after which the 

 boiling would become difficult. On the contrary, the boiling 

 was just as easy as before. 



M. Gernez is so impressed with the necessity for the interven- 

 tion of air in the phenomena we have been considering, that he 

 cannot do without it in accounting for the line of bubbles which 

 is produced by the friction of a chemically clean solid against 

 the inner side of a chemically clean vessel containing a super- 

 saturated gaseous solution or a liquid at or near the boiling- 

 point. I have already endeavoured to give a simple explanation 

 of these phenomena*. " 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." 



M. Gernez says : — " As it results from these divers experiments 

 that in each bubble of vapour disengaged there is always a smali 

 quantity of air, it is natural to suppose that the friction of solid 

 bodies in the middle of a liquid should determine the separation 

 of a small quantity of gas which was in a state of supersaturated 

 solution and which serves as an atmosphere into which the liquid 

 emits vapour." 



Considering how minute a portion of air or gas liquids at or 

 near the boiling-point, and especially after repeated boiling, can 

 hold in solution — considering also that such liquids when removed 

 from the source of heat, can be made to boil up again and even 

 to boil over, by the mere friction of a hard solid, I cannot con- 

 ceive that these multitudes of bubbles are called into existence 

 by first liberating air; but I can conceive that the vapour, being 

 already in solution in the hot liquid, is in a condition to rush into 

 the vacuum formed for it by a solid moving against the side, 



* Phil. Mag. for November 1874, 



