350 



On Supersaturated Saline Solutions. [May 16, 



face and the tension improves ; so that the same surface which may have 

 sufficient tensile force to cause a drop of oil to spread upon it, might some 

 hours earlier have retained it in the lenticular form*. 



There are also certain modifications to which oiis &e. are subject in con- 

 sequence of the presence of ozone and other matters in the air, which may 

 somewhat disturb the results expected to be obtained from the action of 

 surface-tension. 



It was stated in Part II. that when an oil &c. assumes the lenticular 

 form, the solution may be agitated so as to break up the lens into a mul- 

 titude of globules, and give the solution the appearance of an emulsion. 

 In such a case the tensions of the two liquids are of nearly the same value ; 

 if not, the agitation often produces crystallization ; but even in the former 

 case it was stated that a sudden jerk will sometimes produce immediate 

 solidification of the solution. Now taking the tension of the solution at 

 5*2, and that of oil of olives at 3*7, and the tension at the surface of sepa- 

 ration of the solution and the oil-lens at about 2, then the sum 3* 7 + 2 is 

 equal to the tension of the solution, and the spreading on the surface ought 

 to be impossible, unless fine clear weather, absolutely clean vessels and so- 

 lutions, and the absence of surface-viscosity concur to increase the surface- 

 tension of the solution. At the surface of separation .of the solution and 

 of the glass, spreading may be possible in the case of certain oils without 

 these concurring circumstances. Suppose a drop or a minute globule of 

 oil to be brought into direct contact with the wet solid side of the solution, 

 as by the jerk above referred to, the film of solution is displaced and the 

 oil can wet the solid side. It may happen that the tension t of the solution 

 at the wall of the flask is greater than the sum of the tension t of the sur- 

 face of separation of the solution and of the oil, plus the tension of the oil 

 in contact with the solid side ; that being the case, the instant solidifica- 

 tion consequent on the jerk is accounted for. 



It will be seen, then, that when the drop of oil &c. remains as a lens on 

 the surface, there is a diminution of tension at the surface of the solution 

 in contact with the oil ; but in such a case the tension is not sufficiently 

 lowered at one point as to render molecular equilibrium impossible at this 

 point, and so break up the whole system of supersaturation. But if the 

 solution be agitated, so as to bring into contact with the surface of the 

 glass a portion of the drop, there will still be diminution of tension at the 

 surface of the solution in contact with the solid, and now the diminution 

 is sufficient to produce crystallization. Thus it appears that oils may act 



* Some of the distinguished physicists who are now engaged in studying the pheno- 

 mena of surface-tension refer to the embarrassing effects of surface-viscosity. Thus 

 Herr Liidtge remarks that a solution of soap (£=2'8 to 3) does not spread upon a so- 

 lution of Panama-wood (£=5'7) ; and it has been shown by one of us that the viscosity 

 of the surface explains why a solution of soap does not spread on a solution of saponine 

 or of albumen ; and, on the other hand, the liquid drop being viscous, there is no exten- 

 sion, or only a feeble one, since the slight difference in tension is equilibrated by the 

 resistance of the viscous liquid. 



