234 Royal Society : — Mi\ Tomlinson andYan der Mensbrugglie 



so as to break up the oil-lens into a multitude of minute globules, 

 giving the solution the appearance of an emulsion — and that by repose 

 the solution regains much of its transparency, without any separation 

 of salt — but that if, while the flask is being turned round, a sudden 

 jerk be given to it, so as to flatten some of the globules against the 

 side, the solution instantly becomes solid. 



The powerful action of films in putting an end to the state of su- 

 persaturation being thus established, it occurred to one of us, who 

 had already succeeded in explaining a number of obscure phenomena 

 on the principle of surface-tension*, that that force, properly handled, 

 would suffice to account for most, if not all, the varied phenomena of 

 supersaturation. According to this view, whatever tends greatly to 

 lower the surface-tension of a supersaturated saline solution, causes 

 a separation of salt, and at once puts an end to the condition of su- 

 persaturation. 



In order to test this view, a large number of experiments have been 

 performed by one of us during the last six months, consisting of 

 repetitions of former experiments or of new ones suggested by one 

 or both of us. All these experiments have been performed in the open 

 air at Highgate, near London, the object being to avoid all possible 

 miscarriage from the effects of floating dust in the air of a room. It 

 had been suggested that som.e of the former results as to the action 

 of films might have been vitiated from this source ; and although 

 this does not appear to have been the case, yet it is with much satis- 

 faction that the experimenter refers to the greater facility and cer- 

 tainty with which experiments of this kind are conducted in the 

 open air, as compared with those made in a room. In the open air 

 a gentle wind would sometimes blow over the mouths of the flasks, 

 sufficient to produce a low musical note, without any nuclear action, 

 unless a speck of soot or a small insect were carried into the solu- 

 tion ; but in general, in order to prevent evaporation, the flasks were 

 kept covered with watch glasses or small beakers, except when per- 

 forming an experim.ent. 



The salt used in the following experiments was sulphate of soda, 

 in large crystals, not effloresced, one of three strengths being adopted 

 as circumstances required, which will be indicated when necessary 

 — namely, 1 part of salt to 1 of water, 2 parts of salt to 1 of water, 

 and 3 parts of salt to 1 of water. Every solution was first made in a 

 large flask, and filtered boiling into eight or ten small flasks, which 

 were reboiled, covered with watch glasses or beakers, and carried on 

 a tray into the open air. The same experiment was repeated on a 

 number of these solutions of the same strength. 



The points to which this experimental inquiry tended are included 

 in the four following propositions : — 



I. That a supersaturated saline solution contained in a catharized 

 flask will remain liquid so long as its free surface, or the surface in 



* " Sur la Tension superficielle des Liquides,"' par Gr. Yan der Mensbrugglie, 

 Repctiteur a I'Universite de Grand. Memoires couronnes par I'Acad. Eoyale 

 de Belgique, tome xxxiv. 1869. See also Phil. Mag. for Dec. 1869 and Jan. 1870. 



I 



