Mr. T. Graham on Liquid Diffusion applied to Analysis, 369 



known, precipitates lime from the gum very imperfectly. Hydro- 

 chloric acid may be used to separate that base from a solution of 

 gum placed upon the dialyser, with 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 0*1 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 equiva- 

 lent to the lime originally present in the gum (1*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 pot- 

 ash 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-tragacanth. We appear to have here 

 the pectous form of gummic acid. 



It is worthy of inquiry whether such native gums as are in- 

 soluble in water are not the pectous form of soluble gum, rather 

 than allotropic varieties of that substance. So also of the meta- 

 gummic acid of Fremy, formed by the action of strong sulphuric 

 acid on mucilage. This last substance is insoluble in water, but 

 was found by Fremy to afford, when neutralized by lime and 

 alkalies, a soluble gum undistinguishable from gum-arabic. 



Gummic acid produces a remarkable compound with gelatine. 

 When solutions of these two colloids are mixed, oily drops fall 

 and form a nearly colourless jelly on standing. This jelly is 

 very fusible, melting at 25°, or by the heat of the hand. The 

 gummate of gelatine may be washed without decomposition, but 

 is soluble to a certain extent in pure water, and still more so in 

 a solution of gelatine. Prepared with gummic acid in excess, 

 the compound, when dried at 100°, consisted of 100 parts gummic 

 acid with 59 gelatine. The drops and the jelly contained 83*5 

 per cent, of water. Solution of gelatine is not precipitated by 

 unpurified gum, nor by the gummate of potash. 



Dextrine — A two per cent, solution of dextrine, prepared from 

 starch, was diffused in the same conditions as the preceding sub- 

 stances, but through a mucus septum. It gave in twenty-four 

 hours "034 gramme of diffusatc from 2 grammes, or about three 

 times more diffusatc than was given by gum-arabic. 



Caramel. — The dialytic examination of this substance adds to 

 the accurate information on the subject lately supplied by M. A. 

 Gelis*, and places caramel indisputably in the colloid class. 

 * Annates de Chimie, 8fC, ser. 3. t. lii. p. .352. 



Phil. Mag, S. 4. Vol. 23. No. 155. May 1862. 2 C 



