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III. On Supersaturated Saline Solutions. — Part II. By Charles Tomlinson, F.JR.S. 
Received May 17, — Read June 16, 1870. 
I have already in former papers considered the conditions under which gas 1 or steam 2 
or salt 3 is separated from its supersaturated solution, and have endeavoured to show 
that a body is active or inactive as a nucleus, according as it is chemically unclean 
or clean. An objection to these terms has been started, on the ground that a stick of 
tallow, for example, may be as chemically clean as a catharized glass rod. In the first 
Section of this paper an attempt is made to define with rigour the terms clean and un- 
clean , and to settle the conditions on which nuclei really act. In the second place, an 
attempt is made to confirm the conclusion arrived at in Part I., that supersaturation 
depends mainly on the absence of a nucleus, by a number of examples in which highly 
supersaturated saline solutions, when reduced to temperatures at and below the zero of 
Fahrenheit’s scale, rather solidify than crystallize, and in melting reassume the condi- 
tion of clear, bright, supersaturated solutions. 
Section I. — On the Functions of Nuclei. 
I have already endeavoured to show that the obscure and often contradictory be- 
haviour of solids as nuclei in separating gas or vapour or salt from their supersaturated 
solutions, becomes clear by considering whether the solids used as nuclei were or were 
not chemically clean as to surface at the moment of contact with the solution into which 
they were placed. 
A nucleus was defined as a body that has a stronger attraction for the gas or the 
vapour or the salt of a solution than for the liquid that holds it in solution. 
A body is chemically clean the surface of which is entirely free from any substance 
foreign to its own composition. 
It is to be observed that I speak of surface only, and shall hereafter omit the expres- 
sion of surface in referring to nuclei. I call, for instance, a glass rod chemically clean, 
although a particle of carbon or of oxide of iron or other matter be enclosed and shut 
up within it ; but not so if that particle reach and form a portion of the surface itself. 
So, also, I call a stick of tallow, stearine, paraffine, &c. chemically clean, so long as its 
surface falls under the definition just given. 
In like manner, liquid oils, both fixed and volatile, and other liquids are chemically 
clean, provided they are chemically pure, and contain no substance, mixed or dissolved, 
1 Phil. Mag., August and September, 1867. 2 Proceedings of the Royal Society, No. 108, 1869. 
3 Transactions of the Royal Society, 1868, p. 659. 
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