SOME CONSEQUENCES OF GRAHAM'S WORK 611 



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 crystallise 

 when the liquid was evaporated on a water bath." 



The sized paper, Graham points out, has no power to act as 

 a filter. Molecules only, he says, permeate the septum — not 

 masses. But diffusion takes place through the water contained 

 in interspaces in the colloid. He introduced the use of vegetable 

 parchment as a dialysing medium. 



Colloids are substances which diffuse with extreme slowness. 

 Their presence in water opposes little if any hindrance to the 

 diffusion of crystalloids, diffusion taking place as easily from a 

 solid jelly containing several per cent, of gelatin as from a simple 

 aqueous solution. 



Colloid septa are of special interest as the lining membranes 

 of vegetable and animal cells and living tissues generally are of 

 this nature. But the natural septa are far more perfect than 

 Graham's vegetable parchment — they are not permeable by salts 

 and many other crystalloids which pass through such a septum. 

 No better illustration can be given of the part such septa play 

 than that afforded by the barley grain. Obviously, it is impor- 

 tant that the food materials stored up in seeds should not pass 

 outwards during germination and also that harmful materials 

 should not pass in and check or stop growth. Prof. Adrian Brown 

 has shown recently 1 that cereal grains are provided with a lining 

 membrane close to the outer skin of the seed which has 

 most remarkable power as a differential septum. He has 

 demonstrated this in a most beautiful way by taking advantage 

 of the fact that in certain blue barleys the layer referred to 

 contains a colouring matter, resembling litmus, which is reddened 

 by acids. When the barley grain is steeped in water it gradually 

 takes up water to the extent of 70 to 80 per cent, of its weight. 

 It equally absorbs water from dilute solutions of strong acids 

 such as sulphuric acid but may be kept in the acid liquid during 



1 Proceedings of the Royal Society, B. vol. lxxxi. p. 82 (1909). 



