Mr. T. Graham on Liquid Diffusion applied to Analysis. 207 



separated from the other two metals by an application of the jar 

 diffusion above described. 



A certain property of colloid substances comes into play most 

 opportunely in assisting diffusive separations. The jelly of 

 starch, that of animal mucus, of pectine, of the vegetable gelose 

 of Pay en, and other solid colloidal hydrates, all of which are, 

 strictly speaking, insoluble in cold water, are themselves per- 

 meable 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 re- 

 semble animal membrane in this respect. A mere film of the 

 jelly has the separating effect. Take for illustration the follow- 

 ing simple experiment. 



A sheet of very thin and well-sized letter-paper, of French manu- 

 facture, 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 subacetate 

 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 crystallized sugar, but has resisted 

 .the passage 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 mechani- 

 cally impenetrable, and denies a passage to the [mixed fluid 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 cither 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 col- 

 loid, such as starch. The sugar thus obtains the liquid medium 



