534 



Mr. C. Tomlinson on 



[June 16, 



nucleus ; or (2) it will spread out into a film, or (3) remain in a lenti- 

 cular shape. It becomes a film or a lens according to the general propo- 

 sition, 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,"^, is less than 

 a y 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 super- 

 saturated saline solutions, on account of the superficial viscosity, or the 

 greater or less difficulty of the superficial molecules to be displaced. 



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 con- 

 densation of floating vapours, dust, &c, and in either case is brought into 

 the nuclear condition. 



A second class of nuclear bodies are permanently porous substances, 

 such as charcoal, coke, pumice, &c. The action of these is chiefly con- 

 fined to vaporous solutions, and if catharized having no power of sepa- 

 rating salts from their supersaturated solutions. 



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

 are glass, the metals, &c, while their surfaces are chemically clean. 



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

 nuclear character is due, not to itself, but to the nuclear particles 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 solution adheres 

 to it more strongly than the liquid portion, and hence there is a separa- 

 tion. 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 adhesion 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 globule, 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 solutions. 



Other liquids (glycerin 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 clean 

 flasks, and covered with watch-glasses. When cold, the watch-glass 

 being lifted off, a drop of oil is deposited on the surface of the supersatu- 



