on Supersaturated Saline Solutions. 231 



into a multitude 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 crystalli- 

 zation ; 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 separation 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 impos- 

 sible, unless fine clear weather, absolutely clean vessels and solutions, 

 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 mi- 

 nute 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 hap- 

 pen that the tension t of the solution at the wall of the flask is greater 

 than the sum of the tension t of the surface of separation of the so- 

 lution and of the oil plus the tension of the oil in contact with the 

 solid side ; that being the case, the instant solidification 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 sufiiciently lowered at one point to render molecular equili- 

 brium 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 pro- 

 duce crystallization. Thus it appears that oils may act differently 

 according as they alter the tension of the liquid freely exposed to the 

 air, or the tension of the liquid in contact with the glass, which is 

 not of the same value. 



With respect to Proposition III. there is no difficulty. A liquid 

 of considerable contractile force, such as pure water, produces no 

 separation of salt in a solution of less contractile force. This explains 

 a number of cases described in a note by one of us submitted- to the 

 Society in July last*, in which solutions exposed for hours together 

 tp heavy rain did not crystallize, unless the rain brought down a 

 speck of soot or some unclean body that lowered the surface-tension 

 of the solution. Indeed we know of no liquid of superior tensile 

 force to that of the solution, and not acting chemically upon it, that 

 has any influence in producing crystallization. 



Proposition IV. also agrees with the phenomena. A glass rod or 

 other solid, more or less smeared with a film of a liquid of low 

 tension, when brought into contact with the solution determines crys- 

 tallization by lowering the surface-tension. Such, then, is the func- 

 * Proc. Koj. Soc. vol. xx. p. 41. 



