598 WILLIAM LAWTON GOODWIN ON THE 
And Berthollet, in his Statigue Chimique (i. 35), explains solution as 
follows :— 
“Water has obviously an affinity for all those bodies with which it is capable of uniting. 
But affinity is mutual. We may say with as much propriety that the solid acts on the liquid 
as that the liquid acts on the solid. Both act upon each other reciprocally, and at the same 
time ; but the force exerted by each will be proportional to its mass. Now, there is this peculi- 
arity in the action of liquids upon solids, that they can only act at the point of contact, or 
at least near it, Hence, as far as the mass is concerned, it is quite the same thing whether a 
solid be acted on by a large quantity of liquid or by a small quantity, since the points of con- 
tact, and of course the sphere of the liquid’s activity, must in both the cases be the same. 
When a solid body, then, is plunged into a liquid for which it has an affinity, whatever the 
quantity of liquid may be, the action is always limited to a very small portion. Hence the 
liquid is not capable at first of destroying the cohesion of the solid; the latter imbibes it and 
combines with it, while new portions of liquid come into contact, and begin to exert their action. 
If the affinity between the solid and the liquid be weak, the combination proceeds only till the 
force of affinity is so far weakened by the quantity of water united, that it is no longer able to 
overcome the cohesion of the particles of the solid, and then it necessarily stops. The com- 
pound continues solid. With such solids water is capable only of forming a hydrate ; it does 
not dissolve them. If the affinity be strong, new doses of water continue to combine with the 
atoms of the solid, and thus these atoms are separated farther and farther from each other ; 
but as this distance increases the force of cohesion continually diminishes, while the liquid, by 
its increased mass, is enabled to act with greater and greater energy. Hence the cohesion of 
the solid is gradually destroyed; the particles of it are separated to too great a distance, and 
are dispersed equally through the liquid. This is what is termed solution. If we continue to 
add more of the solid after a portion has been dissolved in this manner by the liquid, it will 
be dissolved in the same way. But by this new portion the particles of the dissolved solid are 
brought nearer each other in the solution; their mass is increased in proportion to that of the 
liquid. Hence they exert a greater force on it, and of course the liquid is enabled to exert 
only a smaller force upon new portions of the solid. If we continue to add new portions of 
the solid, a time will come when the action of the liquid will be so much weakened that it will 
no longer be able to overcome the cohesion of the solid, and it will then refuse to dissolve any 
more of it. When a liquid has come to this state, it is said to be saturated with the solid. 
Were we to suppose the solution to go on, the particles of the solid in solution would be 
brought so near each other that their force of cohesion would overbalance the affinity of the 
liquid for them, They would, in part, cohere, and form again a new portion of the solid. The 
saturation of a fluid, then, does not mean that its affinity for the solid is satisfied, but that it 
is not greater than the tendency of the combined particles to cohere. Now, when a liquid is 
saturated with a solid, if by any means we can abstract part of that liquid, the cohesive force 
of the particles of the solid must gain the superiority, and the consequence will be, that they 
will unite and form solid bodies anew, till their number be so diminished that their mutual 
attraction is again counterbalanced by the attraction of the liquid. Hence the reason that 
evaporation occasions the crystallisation of those bodies which are held in solution by liquids. 
If the affinity between water and the solid be not sufficiently great to enable it to overcome 
any part of the cohesion of the particles of the solid, in that case none of it combines with that 
body, it only moistens its surface. If the affinity is even weaker than the cohesion of the 
particles of the liquid, in that case the surface of the solid is not even wetted.” 
