]VIE. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
185 
table gelose of PAYE]!f, and other solid colloidal hydrates, all of which are, strictly speak- 
ing, insoluble in cold water, are themselves permeable when in mass, as water is, by the 
more highly diffusive class of substances. But such jellies greatly resist the passage of 
the less diffusive substances, and cut off entirely other colloid substances like themselves 
that may be in solution. They resemble animal membrane in this respect. A mere 
film of the jelly has the separating effect. Take for illustration the following simple 
experiment. 
A sheet of very thin and well-sized letter paper, of French manufacture, having no 
porosity, was first thoroughly wetted and then laid upon the surface of water contained 
in a small basin of less diameter than the width of the paper, and the latter depressed 
in the centre so as to form a tray or cavity capable of holding a liquid. The liquid 
placed upon the paper was a mixed solution of cane-sugar and gum-arabic, containing 
5 per cent, of each substance. The pure water below and the mixed solution above 
were therefore separated only by the thickness of the wet sized paper. After twenty-four 
hours the upper liquid appeared to have increased sensibly in volume, through the agency 
of osmose. The water below was found now to contain three-fourths of the whole sugar, 
in a condition so pure as to crystallize when the liquid was evaporated on a water-bath. 
Indeed the liquid of the basin was only in the slightest degree disturbed by sub- 
acetate of lead, showing the absence of all but a trace of gum. Paper of the description 
used is sized by means of starch. The film of gelatinous starch in the wetted paper has 
presented no obstacle to the passage of the crystalloid sugar, but has resisted the pass- 
age of the colloid gum. I may state at once what I believe to be the mode in which 
this takes place. 
The sized paper has no power to act as a filter. It is mechanically impenetrable, and 
denies a passage to the mixed fiuid as a whole. Molecules only permeate this septum, 
and not masses. The molecules also are moved by the force of diffusion. But the water 
of the gelatinous starch is not directly available as a medium for the diffusion of either 
the sugar or gum, being in a state of true chemical combination, feeble although the 
union of water with starch may be. The hydrated compound itself is solid, and also 
insoluble. Sugar, however, with all other crystalloids, can separate water, molecule 
after molecule, from any hydrated colloid, such as starch. The sugar thus obtains the 
liquid medium required for diffusion, and makes its way through the gelatinous septum. 
Gum, on the other hand, possessing as a colloid an affinity for water of the most feeble 
description, is unable to separate that liquid from the gelatinous starch, and so fails to 
open the door for its own passage outwards by diffusion. 
The separation described is somewhat analogous to that observed in a soap-bubble 
inflated with a gaseous mixture composed of carbonic acid and hydrogen. Neither gas, 
as such, can penetrate the water-film. But the carbonic acid, being soluble in water, is 
condensed and dissolved by the water-film, and so is enabled to pass outwards and reach 
the atmosphere ; while hydrogen, being insoluble in water, or nearly so, is retained 
behind within the vesicle. 
2 D 
MDCCCLXI. 
