on Gaseous Supersaturated Solutions, 279 



of stearine or paraffin, although chemically clean, become covered 

 with bubbles as soon as they are placed in soda-water. But 

 here other conditions are introduced, such as porosity, and the 

 very different adhesion between resin &c. and air or gas, and 

 resin &c. and water. 



Dr. Henrici's idea of clean surfaces is such that, if previously 

 cleaned according to his method with leather and pumice and 

 exposed to the air, wiping them with a cloth or rinsing them is 

 sufficient to restore their purity. A platinum wire or glass rod 

 so cleaned and immersed in soda-water becomes covered with 

 gas-bubbles ; and this he urges as a proof of the chemical purity 

 of such surfaces, and he even maintains that this is their proper 

 function; whereas the very reverse is the truth. 



Consider the problem in hand*. A supersaturated solution 

 of a gas with its upper surface exposed to the air is always giving 

 Gff gas either with effervescence or silently. It does so because 

 the excess of gas has but a slight adhesion to the liquid, and the 

 air is virtually a vacuum for it. Now the remaining surface of 

 the liquid, or that confined by the sides of the vessel, may be re- 

 garded as being in exactly the same condition, subject, however, 

 to two modifications — (1) the state of chemical purity of their 

 surface, and (2) the pressure exerted by them virtually on the 

 liquid. (1) Suppose the vessel to be of glass, and to be chemi- 

 cally clean according to the definition given above. No gas will 

 be disengaged and no bubbles will form on the sides, because the 

 adhesion between the sides and the gaseous solution is perfect ; 

 and therefore the sides may be regarded, pro rata, as merely a 

 continuation of the liquid itself, and no bubbles will form there 

 any more than in the central parts of the liquid. (2) But sup- 

 pose the* sides to be not chemically clean — to be dirty in fact ; 

 adhesion is diminished or destroyed, and therefore the surface of 

 the liquid next to such sides is virtually as free as its upper sur- 

 face : bubbles will consequently form here just as they do on 

 the upper surface; but in the latter case they do not appear as 

 bubbles (except during effervescence) because there is no pres- 

 sure; the sides do exert pressure, and therefore bubbles are 

 formed. It does not matter whether there be air or not between 

 the sides and the liquid. It is no function of air to induce the 

 liberation of gas or the formation of gas-bubbles. It is really 

 want of adhesion. Now apply this to the case of a catharized 

 glass rod, platinum wire, or a newly fractured surface of jflint. 

 Any one of these placed in the liquid does nothing more than 

 form new sides, as it were, to the vessel ; and its effect is merely 

 that of the sides. If chemically clean, the glass rod &c. will 

 form no bubbles round it ; and hence it is inactive because its 

 * See Phil. Mag. Sept. 1867. 



