3IR. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
215 
It is worthy of inquiry whether such native gums as are insoluble in water are not 
the pectous form of soluble gum, rather than allotropic varieties of that substance. 
So also of the metagummic acid of Fkemy, formed by thn action of strong sulphuric acid 
on mucilage. This last substance is insoluble in water, but was found by Feemy 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 di’ops fall and form a nearly colourless jelly on stand- 
ing. This jelly is very fusible, melting at 25°, or by the heat of the hand. The fftm- 
mate of gelatine may be washed without decomposition, but is soluble to a certain extent 
in pm’e 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 gummic acid mth 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. 
Dextrin . — A two per cent, solution of dextrin, prepared from starch, was diffused in 
the same conditions as the preceding substances, but through a mucus septum. It 
gaA’e in twenty-four hours ’034 gramme of diffusate ffom 2 grammes, or about three 
times more diffusate than was given by gum-ai’abic. 
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. The crude caramel obtained by heating cane-sugar at 210° — 220°, when 
placed on the dialyser, allows certain interaiediate colom’ed substances (Caramelane and 
Caramelene of Gelis) to diffuse out with considerable facility, while the compound con- 
taining the largest proportion of carbon remains behind. The latter substance, as obtained 
by me, possessed five times the colouring power of the original crude caramel, weight 
for weight. This highest soluble member of the caramel series may also be obtained, 
more quickly, by precipitation from its aqueous solution by means of alcohol. But I 
found it necessary to repeat the precipitation four times, or till the mass thrown down, 
from being plastic at first became pulverulent. A solution containing 10 per cent, of 
the caramel so purified is gummy; and on standing, it formed a tremulous jelly entirely 
soluble in hot or cold water. Evaporated in vacuo, the solution dries up into a black 
shining mass, which is tough and elastic, while it still possesses a certain proportion 
of water, like gum containing some water. Once thoroughly dried at a low temperature, 
this soluble caramel may be heated, afterwards, to 120° and retain complete solubility. 
But if a solution of the same caramel be directly evaporated to dryness by the heat of a 
water-bath, the whole matter is rendered insoluble in hot or cold water. The soluble 
and insoluble caramel have the same composition, and appear to illustrate the usual 
double form of colloids. The proportion of carbon in the fluid caramel was found as 
high as 54'59 per cent., which comes nearer to C 24 Hi 5 0jg (requiring C55T7) than any 
other formula in which the oxygen and hydrogen are assumed to be present in the pro- 
* Annales de Chimie, &c., ser. 3. t. lii. p. 352. 
