n.] THE ALBUMENOIDS. I3 



LESSON n. 



THE ALBUMENOIDS. 



The group of albmnenoids includes a number of bodies which 

 in their general characters and elementary composition resemble 

 proteids, but differ from them in many respects. They are amor- 

 phous. Some of them contain sulphur, and others do not. The 

 decomposition-products resemble the decomposition-products of 

 proteids. 



1. I. Gelatin is obtained by the prolonged boiling of connective 

 tissues, e.g.^ tendon, ligaments, bone, and from the substance 

 " Collagen," of which fibrous tissue is said to consist. 



Preparation of a Solution. — Make a watery solution (5 per 

 cent.) by allowing it to swell up in water, and then dissolving it 

 with the aid of heat. 



(A.) (c/.) It is insoluble, but swells up in about six times its 

 volume of cold water. 



{h.) After a time heat the gelatin swollen up in water; it dis- 

 solves. Allow it to cool ; it gelatinises. 



(B.) With General Proteid Tests. 



(c.) Xanthoproteic Test.— Add nitric acid and boil = a Hglit 

 yellow colour with no previous precipitate; the fluid becomes 

 orange or rather lemon-coloured on adding ammonia. 



(d.) Millon's Eeagent = no pinkish-red precipitate on boihng. 

 This shows the absence of the tyrosin group in the gelatin molecule. 

 This reaction may be obtained with commercial gelatin, but not 

 with pure gelatin, so that the reaction if obtained is due to 

 impurities. 



{e.) It gives a blue- violet, rather than a violet colour, with NaHO 

 and CUSO4. 



(/■.) It is not precipitated by acetic acid and potassic ferrocyanide 

 (unlike albumin). 



{g.) It is not coagulated by heat (unlike albumin). 



{h.) It is not coagulated by boiling with sodic sulphate and acetic 

 acid (unlike albumin). 



(i.) It is precipitated by saturation with MgSO^ or (^11^)2804. 



(C.) Special Reactions. 



(y.) It is not precipitated by acids (acetic or hydrochloric), or 

 alkalies, or lead acetate. 



(A*.) Add mercuric chloride = no precipitate (unlike albumose and 

 peptone). 



