456 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Hydroferrocyanic acid precipitates it only under certain conditions. 

 Tannic acid gives a copious and voluminous precipitate. 



Gelatin is not precipitated by saturation with sodium chloride, but 

 comes down on acidifying. 



Gelatin is precipitated from solution by saturation with magnesium 

 sulphate or by half-saturation with ammonium sulphate. 



Other Scleroproteins in Vertebrate Animals. 



Several other scleroproteins have been described, but have been little in- 

 vestigated ; they are : 



Reticulin of the reticular connective tissue in lymph glands, liver and 

 kidneys. 



Albumoid of the lens. 



Albumoid of cartilage. 



Albumoid of bone or osseoalbumoid. 



Membranin of the capsule of the lens. 



Scleroproteins of Invertebrate Animals. 



Silk consists of two proteins, silk- fibroin and silk-gelatin; or sericin. Silk 

 is heated two or three times for 3 hours at 117-120 with 25 times its weight of 

 water. Silk-gelatin goes into solution, leaving the fibroin. 



Fibroin has the appearance of silk, but is not glossy and is insoluble. Of 

 the colour reactions for proteins it gives the Millon and biuret reactions. 



Silk-gelatin resembles ordinary gelatin. 



Spider's silk closely resembles fibroin and contains no silk-gelatin. 



Conchiolin is the insoluble protein in the shell of molluscs. 



Gorgonin is the insoluble protein in corals. 



Spongin is the insoluble protein of sponges and contains iodine. 



