ME. CHAELES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 
A 9 
that is foreign to their composition. But in considering such bodies, there is a distinction 
to be made with respect to their nuclear functions, whether they exist in the mass, such 
as a lens or globule, or in the form of thin films. 
Catharization 4 is the act of clearing the surface of bodies from all alien matter ; and 
the substance is said to be catharized when its surface is so cleared. 
As every thing exposed to the air or to the touch takes more or less a deposit or film 
of foreign matter, substances may be conveniently classed as catharized or uncatlmrized 
according as they have been, or not, so freed from foreign matter. 
And it is perhaps not taking too much licence with language to extend the term 
catharized (denoting, as it does, the condition of pure surface) to those substances whose 
surface lias not required the process. Thus a flint stone in the rough has an uncatha- 
rized surface ; but split it and the inner surface of the pieces will, for a time, be clean. 
There can be no impropriety in speaking of the new surfaces as in a catharized or che- 
mically clean state. 
Referring to the definition of a nucleus, substances may be divided into nuclear and 
non-nuclear . 
The nuclear are those that may, 'per se, become nuclei ; the non-nuclear are those 
that have not that quality. 
The nuclear substances would seem to be comparatively few, the larger number of 
natural substances ranking under the other division. 
Under nuclear substances are included those vapours and oily or other liquids that 
form thin films on the surfaces of liquids and solids ; and generally all substances in the 
form of film, and only in that form. Thus a stick of tallow, chemically clean, will not 
act, but a film of it will act powerfully ; and, again, a globule of castor-oil will not act 
if chemically clean, but in the form of a film, whether chemically clean or not, it will 
act powerfully. 
If a drop of a liquid be placed on the surface of another liquid, it may mingle with 
it ; or it will either spread out into a film or remain in a lenticular shape, according to 
the general proposition that if a drop of a liquid B, whose surface-tension is h, be placed 
on the surface of another liquid A, whose surface-tension is a, the drop will spread into 
a film, if a>b-\-c ( c being the tension of the common surface of the liquids A and B) ; 
but if, on the contrary, a— < b-\-c, 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. c has no value whenever the 
liquids A and B mingle in all proportions, as in the case of water and alcohol. 
In the case of supersaturated saline solutions, the spreading of the drop may in some 
cases be slow, 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 condensation of floating vapour, dust, 
&c., and in either case is brought into the nuclear condition. 
4 Erom xaflap? w, to purge, purify, or clean — from xaflajoj, pure, clean. 
