I. HISTORICAL. 569 



same manner as between gases, except that the rate of diffusion, -which varies 

 with the nature of the liquids, the temperature, and the degree of concentra- 

 tion, is slower. The phenomena are governed by several well ascertained 

 laws, for a brief account of which see Watts's Dictionary of Chemistry, 

 Vol. III., p. 705. 



2541. Osmometers, or apparatus employed in Graham's re- 

 searches on " Osmotic Force." 



When a solution of a salt in a liquid is separated by a membrane or porous 

 septum from a mass of water, a flow of liquid takes place from one side of 

 the septum to the other. In some cases this flow is sufficiently powerful to 

 sustain a column of water several inches in height. This diffusion is termed 

 " Osmose." The experiments of Graham led to the conclusion that osmose 

 depends essentially on the chemical action of the liquid on the septum. The 

 base of the bell glass is covered either with thin unglazed earthenware or 

 animal membrane, and is of such a size that the area of the opening is 100 

 times that of the tube, in order to facilitate the observations. 



2542. Apparatus employed in experiments on Liquid Dif- 

 fusion applied to analysis and on dialysis. 



It consists simply of a gutta-percha hoop, about 9 inches in diameter, on 

 which is stretched a sheet of parchment paper. The solution to be dialysed 

 is placed in the hoop, and the whole floated in a considerable volume of water. 

 Animal membrane, or a mere film of a colloidal septum, permits crystalloids 

 to diffuse freely through it, but is entirely impervious to any colloids, for 

 example gelatine, which may be in solution. The septum does not act in any 

 way as a filter, but permits only the permeation of molecules and not masses. 

 This mode of diffusion Graham termed Dialysis. By its aid he obtained 

 hydra ted silicic acid, oxides of iron and alumina, &c., all soluble in water, 

 and he succeeded in separating crystalloid poisons from organic matter in 

 toxicological investigations. 



2543. Apparatus by which Graham studied Capillary 

 Liquid Transpiration in relation to chemical composition. 



The globe was filled with the liquid under examination, and the force em-- 

 ployed to impel it through the capillary was the weight of 1 atmosphere, and 

 was obtained from compressed air contained in the large reservoir, which was 

 provided with a mercurial gauge. 



2544. Tube Atmolyser, or instrument for the separation 

 of Gases by diffusion. 



As gases differ in the rate at which they transpire through a porous septum, 

 it follows that, when a mixture of gases is in contact with a septum, while the 

 other side is vacuous, the per-centage composition will be changed. The 

 atmolyser consists of a bundle of tobacco-pipe stems, enclosed in a glass tube, 

 which can be rendered vacuous. The mixed gases are passed through the 

 tube. For example, an explosive mixture of 66 per cent, by volume of 

 hydrogen and 33 per cent, of oxygen was passed through the tube. The 

 resulting mixture contained only 9* 3 per cent, of hydrogen, and a taper 

 burned in it without explosion. 



2545. Barometrical Diffusiometer, used for the investiga- 

 tion of the Molecular Mobility of Gases. 



The gas under examination was allowed to enter either a perfect or partial 



