222 Royal Society ; — 



will do one of three things (apart from chemical action) : (1) it 

 will diffuse through the liquid, and in general, under such circum- 

 stances, not act as a nucleus ; or (2) it will spread out into a film, 

 or (3) remain in a lenticular shape. It becomes a film or a lens ac- 

 cording to the general proposition that, if on the surface of the liquid 

 A, whose surface-tension is a, we deposit a drop of the liquid B, 

 whose surface-tension, b, is less than a, the drop will spread into a 

 film ; but if, on the contrary, b be greater than a, or only a little 

 less, the drop will remain in the form of a lens. Hence if B spread 

 on A, A will not spread on the surface of B. 



This general proposition may not always apply in the case of 

 supersaturated saline solutions, on account of the superficial viscosity, 

 or the greater or less difficulty of the superficial molecules to be dis- 

 placed. 



A glass rod drawn through the hand becomes covered with a 

 thin film ; or the same rod by exposure to the air contracts a film by 

 the condensation of floating vapours, dust, &c. ; and in either case it 

 is brought into the nuclear condition. 



A second class of nuclear bodies are permanently porous sub- 

 stances, such as charcoal, coke, pumice, &c. The action of these is 

 chiefly confined to vaporous solutions ; and if catharized they have no 

 power of separating salts from their supersaturated solutions. 



Under the non-nuclear, forming by far the larger class of sub- 

 stances, are glass, the metals, &c, while their surfaces are chemically 

 clean. 



Among the non-nuclear substances will be found air ; for its as- 

 cribed nuclear character is due, not to itself, but to the nuclear par- 

 ticles of which it is the vehicle. Thus, as stated in Part I., if air be 

 filtered through cotton- wool it loses its apparent nuclear character ; 

 so also if heated. 



When a catharized body is placed in a supersaturated solution, 

 such solution, as explained in Part I., adheres to it as a whole ; but 

 if such body be non-catharized, the gas or vapour or salt of the so- 

 lution adheres to it more strongly than the liquid portion, and hence 

 there is a separation. In the present paper it is shown that an 

 active or non-catharized surface is one contaminated with a film of 

 foreign matter, which filmy condition is necessary to that close ad- 

 hesion which brings about the nuclear action ; for it can be shown 

 that an oil, for example, is non-nuclear in the form of a lens or glo- 

 bule, but powerfully nuclear in the form of a film. 



Some liquids (absolute alcohol for example) form films, and act 

 as nuclei by separating water instead of salt from supersaturated so- 

 lutions. 



Other liquids (glycerine for example) diffuse through the solutions 

 without acting as nuclei. 



Fatty oils may slowly saponify, or oil of bitter almonds form 

 benzoic acid in contact with supersaturated solutions of Glauber's 

 salt without acting as nuclei. 



The solutions (say of Glauber's salt) are prepared with 1, 2, or 

 3 parts of the salt to 1 part of water ; they are boiled, filtered into 



