96 - REPORTS ON THE STATE OF SCIENCE.—1917. 
to 3-25 according to the concentration of the solutions and the time, 
and gives the following figures showing the composition of the precipitate : 
Unwashed. Washed. 
yelatin . F a 725 32 
Tannin . é seats . 68 
According to Trunkel (‘ Biochem. Zeitschr.’ 1910, 26, 458-492), the 
reaction between tannin and gelatin is an adsorption phenomenon. Under 
certain conditions the whole of the gelatin and tannic gcid can be pre- 
cipitated quantitatively, the product then being insoluble and resistant 
to water. With excess of tannin the amount of tannin may be three 
times that of the gelatin, but the precipitate is then unstable and affected 
by water. By continued treatment with alcohol, 97 per cent. of the tannin 
can be removed, the remainder being retained. From the residue only 
about 6 per cent. of gelatin could be obtained in an unaltered condition. 
Trunkel also states that, with a freshly prepared solution of tannin, 1 part 
of gelatin required 0-7 part of tannin for complete precipitation, whereas 
with a solution 24 hours old only 0-4 part was required. 
H. R. Procter \‘ Jour. Soc. Chem. Ind.’ 1910, 329) also states that the 
precipitate is a colloidal combination, not a truly chemical (ionic) re- 
action, the precipitate is not definitely quantitative, and that the tannin 
can be removed to an almost unlimited extent by washing with water. 
Gelatin in dilute solution, mixed with Japanese isinglass and also 
with agar, gives a somewhat similar reaction to that with gum arabic, 
a turbidity appearing which is increased on acidifying with HCl. 
Chondrin from cartilage, although similar in its properties to gelatin, 
does not form such a stiff gel on cooling, it is also precipitated by alum and 
other salts which do not precipitate gelatin (Church, ‘ The Chemistry of 
Paints and Painting,’ 1892, p. 65). Isinglass is somewhat similar to 
gelatin in its properties although it is not so easily soluble; it is used in 
* fining’ wine in which case it 1s coagulated, and carries down with it the 
finely divided suspended matter which rendered the liquid turbid. 
The properties of gelatin are very considerably affected by impurities 
(decomposition products = gelatin peptone, &c.) and according to C. 
Dhéré and M. Gorgolewski (‘Compt. Rend.’ 1910, 150, 934-936) the 
latter can be almost entirely removed by dialysis over a long period, 7.e., 
14 to 3 months, or by freezing a weak solution, when flocks of demineralised 
gelatin separate. Commercial gelatin carries a positive charge while 
demineralised gelatin is negative, therefore pure gelatin will be electrically 
neutral. Gelatin purified by these methods yields opalescent sols within 
certain limits of concentration. At 2 per cent. the sol is turbid, at 8 per 
cent. nearly clear, and at 10 per cent. quite so. Addition of alkali either 
partially or entirely removes the turbidity, but acids have not such a 
strong effect. A solution of such purified gelatin does not form such a 
strong gel as the original, a flocculent product ‘settling out. Traces of 
electrolytes, t.e., acids, alkalies, and salts, cause gelatinisation to take 
place; this is the case with salts like KI which have an opposite effect 
upon ordinary gelatin‘ 
According to J. T. Wood (‘ Collegium,’ 1908, 12, 494-5), the removal 
of the salts or the precipitation of lime by ammonium oxalate affects the 
tannin precipitate, rendering it less in amount. “He is therefore of opinion 
that the predominant ions are those of calcium. Meunier states that a 
