80 GELATIN. 



to prolonged boiling with water, more especially under pressure as in a 

 Papin's digester, they are gradually dissolved, and the solution now 

 contains true gelatin into which they have been converted by hydrolysis, 

 and has acquired the characteristic property of solidifying into a jelly 

 on cooling. The conversion of collagen into gelatin may be still more 

 easily effected by a shorter boiling in presence of dilute acids, but in 

 this case, unless the process be carefully regulated, the first-formed 

 gelatin is further hydrolysed into what are often spoken of as gelatin- 

 peptones. Although insoluble in dilute acids collagen is readily dis- 

 solved by digestion with pepsin in presence of an acid passing rapidly 

 through the condition of gelatin into that of gelatin-peptone, and 

 although collagen is not acted upon by trypsin in alkaline solution, it 

 is readily hydrolysed by this enzyme after a short preliminary treat- 

 ment with dilute acid or boiling water, the products as before being 

 known as gelatin-peptones. When gelatin is exposed for some time in 

 the dry condition to a temperature of 130 it is reconverted into a 

 substance closely resembling collagen, which may be again converted 

 into gelatin by treatment with water under pressure at 120 01 . 



Gelatin obtained by the above means from connective tissue or 

 bones is, when dry, a transparent, more or less coloured and brittle 

 substance 2 . It is insoluble in cold water, but swells up into an elastic 

 flexible mass which now dissolves readily in water when warmed. 

 When the solution is again cooled it solidifies characteristically into a 

 jelly even when it contains as little as 1 p. c. of gelatin ; it is also 

 readily soluble in the cold in dilute acids and alkalis. The proteid 

 reactions of gelatin are so feeble that they must be regarded as due 

 entirely to unavoidably admixed traces of proteid impurities; more 

 particularly is it to be noticed that the usual reaction of proteids with 

 Millon's reagent is entirely wanting, a fact which indicates the probable 

 absence of aromatic (benzol) residues in its molecule and corresponds 

 to the absence of tyrosin among the products of its decomposition. 

 Notwithstanding that it is in no sense a proteid its percentage com- 

 position approximates to that of the latter class of substances and 

 may be taken as C = 50'76, H = 7'15, O = 23-21, N = 18-32, from which 

 it appears to contain distinctly less carbon than do the proteids ; 

 it is also stated to contain no sulphur when pure, but ordinarily 

 it contains a small amount (-5 p. c.) 3 . Gelatin is precipitated by but 

 few salts, viz. : mercuric chloride and the double iodide of mercury 



1 Hofmeister, Zt. f. physiol. Chem. Bd. n. (1878), S. 313. Weiske, Ibid. vn. 

 (1883), S. 460. 



2 Pure gelatin is colourless, e.g. fine isinglass prepared from the bladder of the 

 sturgeon. Glue is impure gelatin made from hides &c. 



3 Hammarsten, Zt. f. physiol. Chem. Bd. ix. (1885), S. 305. 



