22 



The principal albuminoids are keratin, elastin, collagen, 

 reticulin, and skeletin. 



Keratin occurs in the horny portions of the skin and its 

 appendages. 



Elastin occurs in connective, especially yellow elastic, 

 tissue. 



Collagen includes ossein the chief organic constituent of 

 bone ; chondrigen of cartilage is a collagen mixed with a small 

 quantity of other material. On boiling with water, more 

 readily with very dilute acid, coUagens are converted into 

 gelatin. 



Gelatin is obtained by the prolonged boiling of connective 

 tissues, for example, tendon, ligaments, bone, as well as from 

 the substance collagen. Gelatin is a colorless or straw-colored 

 solid, usually occurring in flakes or sheets, swells with water, 

 and when heated dissolves, forming a clear solution, with the 

 property of preventing the formation of precipitates by hold- 

 ing them in suspension in a finely divided condition ; so that 

 they pass through filter paper. 



30. Make a watery solution of gelatin (5%) by allowing 

 it first to swell up in the cold water, and then dissolving it 

 with the aid of heat. It is insoluble, but swells up in about 

 six times its volume of cold water. 



31. After dissolving with the aid of heat, allow a small 

 portion to cool ; it gelatinizes. 



32. Apply the xanthoproteic test for proteids to some of 

 the dissolved portion ; make notes of any differences as com- 

 pared with proteids in this and in the following tests : 



33. Use Millon's reagent. 



34. Try Piowtrowski's reaction. 



35. Is it precipitated by the acetic acid and potassium 

 ferrocyanide test ? 



36. Does it coagulate by heat ? 



37. Is it precipitated by saturation with magnesium 

 sulphate ? 



38. What is the result of the addition of tannic acid ? 



39. Add picric acid (saturated solution"); if a precipitate 

 appears apply heat and note any change that may occur upon 

 cooling. 



