THE CHEMICAL COMPOSITION OF THE fcODY. 121 



in cold water, but swells up to about six times its volume : it dissolves 

 readily on the addition of very dilute acids or alkalies. It is soluble in 

 hot water, and forms a jelly on cooling, even when only 1 per cent of 

 gelatin is present; it is also soluble in hot salt solution. Prolonged 

 boiling in dilute acids, or in water alone, destroys this power of forming 

 a jelly on cooling. Its physical properties seem to indicate a closer rela- 

 tionship to albumin than to keratin, but decomposition proves the reverse. 

 On decomposition it gives 2 per cent of leucin and 2. 6 per cent of argenin, 

 but no tyrosin ; instead there is a large amount of glycocoll (amido-acetic 

 acid or glycin), a crystalline substance. 



A fairly strong solution of gelatin 2 per cent to 4 per cent gives 

 the following reactions : 



(A) With proteid tests : (i.) Xanthoproteic test. A yellow color but 



no previous precipitate with nitric acid, becoming darker on the 

 addition of ammonia, (ii.) Biuret test. A blue color, (iii.) 

 Millon > s test A pink color but no precipitate, (iv.) Potassium 

 ferrocyanide and acetic acid. No reaction, (v.) Boiling with 

 sodium sulphate and acetic acid. No reaction. 



(B) Special reactions : (i.) No precipitate with acetic acid, (ii.) No 



precipitate with dilute hydrochloric acid, (iii.) A white pre- 

 cipitate with tannic acid, not soluble in excess or in dilute ace- 

 tic acid, (iv.) No precipitate with mercuric chloride, unlike 

 the reaction with albumose and peptone, (v.) A white precipi- 

 tate with alcohol. (vi.) A yellowish-white precipitate with 

 picric acid, dissolved on heating and reappearing on cooling. 

 Collagen is insoluble in almost everything. 



Elastin is found in elastic tissue, in the ligamenta subflava, ligamen- 

 tum nuchae, etc. It is insoluble in all ordinary reagents, but swells up 

 both in cold and hot water. Is soluble in strong caustic soda slowly, 

 when heated. It is precipitated by tannic acid; does not gelatinize. 

 Gives the proteid reactions with strong nitric acid and ammonia, and im- 

 perfectly with Millon's reagent. On decomposition it gives 4.5 per cent 

 of leucin, a small amount of argenin, and a mere trace of tyrosin. It is 

 prepared by boiling with water, then treating with artificial gastric and 

 pancreatic juices, then boiling again in water, and then extracting with 

 acids, alcohol, and ethers ; the remainder is elastin. 



Chondrin is found in the condition of chondrigen in cartilage. 

 It is a mixture of gelatin with a mucin-like substance, and is obtained 

 from chondrigen by boiling. 



Properties. It is soluble in hot water, and in solutions of neutral 

 salt, e.ff., sulphate of sodium, in dilute mineral acids, caustic potash, and 

 soda. Insoluble in cold water, alcohol, and ether. It is precipitated 

 from its solutions by dilute mineral acids (excess redissolves it), by alum, 



