1200 GELATIN. 



collagen is readily dissolved by digestion with pepsin in pres- 

 ence 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, if is readily hydrolyzed 

 by this enzyme after a short preliminary treatment 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. 



Gelatin obtained by the above means from connective tissue 

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

 brittle substance. 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 par- 

 ticularly is it to be noticed that the usual reaction of proteids 

 with Millon's reagent is entirely wanting, a fact which indi- 

 cates the probable absence of aromatic (benzene) residues in its 

 molecule and corresponds to the absence of tyrosine among the 

 products of its decomposition. Gelatin is precipitated by but 

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

 mercury and potassium in acid solution. Several acids on the 

 other hand precipitate it readily, such as phosphotungstic and 

 metaphosphoric, also taurocholic and tannic. Of the two last- 

 named acids the former yields an opalescence in presence of 

 1 part of gelatin in 300,000 of solution, and the latter in still 

 more dilute solutions. 



When decomposed in sealed tubes with caustic-baryta gelatin 

 yields on the whole the same products as do the proteids, with 

 the exception of tyrosine ; neither this nor any other substance 

 of the typically aromatic series, is ever obtained during any 

 decomposition of gelatin whether by chemical or putrefactive 

 processes. By prolonged boiling with hydrochloric acid it 

 yields glycine (glycocoll), leucine, glutamic acid and ammonia, 

 and with sulphuric acid aspartic acid as well. 



Grelatin-peptones. By prolonged boiling with water (1 p.c. 

 solution boiled for 30 hours), or shorter treatment in a Papin's 

 digester, also by heating with hydrochloric acid (4 p.c. at 40), 

 or still more readily by pepsin in presence of acid or by trypsin, 

 gelatin loses its power of solidifying on cooling, and is converted 

 into more highly soluble and now diffusible substances, to which 

 the name of gelatin-peptones has been given. A similar change 



