24 
PROFESSOR GRAHAM ON THE DIFFUSION OF LIQUIDS. 
(3.) A solution of 4 parts of nitrate of potash to 100 of water, of density 1’0241, 
placed in the solution phial, was allowed to communicate with w’ater containing 4 per 
cent, of nitrate of ammonia in the water-jar, which last solution was of density 1’0136; 
with all other circumstances as before. With one solution phial having the usual 
aperture, 1*175 inch, the diffusion product was 15*32 grs. of nitrate of potash. With 
a second phial, having a larger aperture of 1*190 inch, the diffusion product was 18*03 
grs. of nitrate of potash. No comparative experiment, on the diffusion of nitrate of 
potash into water, was made at the same time. But nitrate of ammonia, which ap- 
peared before to coincide in diffusibility with nitrate of potash, gave on a former 
occasion, in similar circumstances, and at 64°*9, nearly the same temperature, a dif- 
fusion product of 15*80 grs. The quantity of nitrate of potash (15*32 grs.) which 
diffused into the solution of nitrate of ammonia approaches so closely to the number 
quoted, that we may safely conclude that the diffusion of nitrate of potash is not 
sensibly resisted by nitrate of ammonia, although these two salts are closely isomor- 
phous. They are still therefore inelastic to each other, like two different gases. 
These experiments have been made upon dilute solutions, and it is not at all im- 
possible that the result may be greatly modified in concentrated solutions of the same 
salts, or when the solutions approach to saturation. But there is reason to appre- 
hend that the phenomena of liquid diffusion are exhibited in the simplest form by dilute 
solutions, and that concentration of the dissolved salt, like compression of a gas, is 
attended often with a departure from the normal character. 
On approaching the degree of pressure which occasions the liquefaction of a gas, 
an attraction appears to be brought into play, which impairs the elasticity of the gas; 
so on approaching the point of saturation of a salt, an attraction of the salt molecules 
for each other, tending to produce crystallization, comes into action, which will inter- 
fere with and diminish that elasticity or dispersive tendency of the dissolved salt 
which occasions its diffusion. 
We are perhaps justified in extending the analogy a step further between the cha- 
racters of a gas near its point of liquefaction and the conditions which we may assign 
to solutions. The theoretical density of a liquefiable gas may be completely disguised 
under great pressure. Thus, under a reduction by pressure of 20 volumes into 1, 
while the elasticity of air is 19*72 atmospheres, that of carbonic acid is only 16*70 at- 
mospheres, and the deviation from their normal densities is in the inverse proportion. 
Of salts in solution the densities may be affected by similar causes, so that although 
different salts in solution really admit of certain normal relations in density, these 
relations may be concealed and not directly observable. 
The analogy of liquid diffusion to gaseous diffusion and vaporization is borne out 
in every character of the former which has been examined. Mixed salts appear to 
diffuse independently of each other, like mixed gases, and into a water atmosphere 
already charged with another salt as into pure water. Salts also are unequally diffusible, 
like the gases, and separations, both mechanical and chemical (decompositions), are 
