214 
ME. T. GRAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
depth of 10 millimetres, was diffused at 10° to 13° of temperature. 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 difPu- 
sates 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 gelatin, and 
therefore contained tannin unaltered. But the diffusates probably contained also 
throughout some products of decomposition of a crystalloid character. 
To the low diffusibility of tannin may be ascribed the remarkably 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 (Feemt). 
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 until the gum, the diffusion of the latter appears to be still further reduced, and 
may even be enthely extinguished. The separation, of colloids from crystalloids 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 Fe^my 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. Oxahc 
acid, it is known, precipitates lime from the gum very imperfectly. Hydrochloric acid 
may be used to separate that base from a solution of gum placed upon the dialyser, ivith 
more effect. It is only necessary to add to a strong solution of gum 4 or 5 per cent, of 
hydrochloric acid, and to dialyse till the gum solution gives no precipitate with nitrate 
of silver. In an experiment made upon a 20 per cent, solution of gum, the ash was 
reduced to O'l per cent, of the gum in five days. The gummic acid possesses a sensible 
acid reaction, about equal to that of carbonic acid. This acid reaction was neutralized 
in 100 parts of gummic acid by 2'85 parts of potash. This amount of potash is very 
nearly equivalent to the lime originally present in the gum (I’72 lime, or 3*07 carbonate 
of lime, being equivalent to 2-89 potash). When the gummate of potash itself was 
dialysed without addition, the potash gradually diffused away, possibly in the state of 
carbonate, and left the gum again possessed of an acid reaction. Gummic acid, well- 
dried at 100°, becomes insoluble in water, but swells up in that liquid, like gum-traga- 
canth. We appear to have here the pectous form of gummic acid. 
