connected with the Boiling of Liquids. . 4<&7 



immediately covered with bubbles of gas or of vapour. These 

 bubbles escape from its surface so long as it continues to be 

 covered, more or less, with a film of a body that can be touched 

 by gas or vapour and not by water. A flint pebble that has 

 been exposed to the air of a room or handled and put into a so- 

 lution of gas or of vapour, immediately becomes covered with 

 bubbles ; but if broken in half and returned to the solution, not 

 a single bubble is to be seen on the fractured parts ; for these are 

 specimens of nature's clean surfaces'*. If air has any action to 

 perform in the matter, why should the unclean and not the clean 

 surfaces carry it down ? 



Some liquids contain their own nuclei, as in the case of milk. 

 When this is heated over the fire it becomes more and more 

 charged with vapour; and at a certain point the particles of 

 butter disseminated through it, assisting the expansive force of 

 the heat, produce such a sudden burst of vapour as to cause the 

 liquid to boil over. 



If a body, such as a glass rod, be made chemically clean and 

 then be plunged into a supersaturated solution of gas or of va- 

 pour, not a single bubble will be seen upon it, because the solu- 

 tion, whether of water and gas, or water and vapour, adheres to 

 it perfectly. If the clean but wet glass rod be left to dry in the 

 dusty air of the room, and when dry be plunged into the solution, 

 it will be active; but if left to dry in the pure outer air of the 

 country, and when dry be plunged into the solution, it is inac- 

 tive, because it is still in a clean or catharized state. 



But it is contended by M. Gernez, and before him by Herr 

 Schroder, that the effect of washing the glass rod or other solid 

 surface in sulphuric acid, or in a caustic alkaline solution, is to 

 get rid of the film of air that adheres to it ; while Herr Schroder 

 admits that unclean bodies act because they are covered more or 

 less with a film of fatty organic matter, but according to him it 

 is this film which enables the air to adhere to the solid. This 

 is the critical point of the inquiry. J say that when a clean glass 

 rod or wire &c. is drawn through the hand and immediately 

 immersed in a supersaturated solution, the gas or the vapour is 

 in a condition to leave the solution and adhere to the unclean 

 surface. If an unclean glass rod be heated and introduced 

 below the surface of the solution, and by moving it against the 

 clean side of the vessel leave a greasy line, such line becomes 

 instantly covered with gas- or vapour-bubbles. A rod or wire 

 heated in the flame of a spirit-lamp not sufficient to clean it, 



* M. Gernez broke a Prince Rupert's drop below the surface of a super- 

 heated liquid; the fragments did not act because, according to him, they 

 contained no air. I say they were inactive because they were chemically 

 clean. ... 



