[ 368 ] 



LI II. Liquid Diffusion applied to Analysis, 



By Thomas Graham, F.R.S., Master of the Mint. 



[Concluded from p. 30G.] 



5. Diali/sis of Organic Colloid Substances. 



finANNIN. — The tannin employed was that extracted from gall- 

 •■*■ nuts by the ether process of Pelouze. A two per cent, solution 

 of this substance, covering a surface of paper-parchment of the 

 area of about -j-J- th of a square metre, or 15*6 square inches, to 

 a depth of 10 millimetres, was diffused at 10° to 13° of tempera- 

 ture. The diffused matter amounted, in successive periods of 

 twenty-four hours, to '073, -040, '021, '021, -024 and -024 

 gramme, derived from the two grammes in solution. Probably 

 the earlier diffusates were increased by the presence of a little 

 gallic acid, which, being a crystalloid, would no doubt be rapidly 

 eliminated by diffusion. The latter observations indicate that 

 tannin passes through a paper-parchment septum about 200 times 

 less rapidly than chloride of sodium does, in similar circumstances 

 as to temperature and strength of solution. The diffusates from 

 the tannin solution gave a precipitate with gelatine, and therefore 

 contained tannin unaltered. But the diffusates probably con- 

 tained also throughout some products of decomposition of a 

 crystalloid character. 



To the low diffusibility of tannin may be ascribed the remark- 

 ably slow penetration of skins by that substance in the ordinary 

 operation of tanning leather. Tannin appears to form compounds 

 of much stability with certain other colloids, as tanno-gelatine, 

 and the compound with albumen which appears to be the primary 

 basis of the vegetable cell (Fremy) . 



Gum. — The diffusate obtained from a solution containing 2 

 grammes of gum-arabic, in experiments corresponding in their 

 conditions with the experiments upon tannin just related, was 

 *013 gramme per day. The power of gum to penetrate the 

 colloid septum appears, therefore, to be one-half less than that 

 of tannin, and 400 times less than the diffusibility of chloride of 

 sodium. Gum gave the same amount of diffusate with a mucus 

 septum as with parchment-paper. When substances of the 

 crystalloid class are mixed with the gum, the diffusion of the 

 latter appears to be still further reduced, and may even be 

 entirely extinguished. The separation of colloids from crystal- 

 loids by dialysis is, in consequence, generally more complete 

 than might be expected from the relative diffusibility of the two 

 classes of substances. 



Vegetable gum, which Fremy has shown to be a gummate of 

 lime, can be purified by a dialytic method, which may be found 

 applicable with advantage in other cases. Oxalic acid, it is 



