I 1O PHYSIOLOGICAL CHEMISTRY. 



ness a hard mass is obtained which is quite insoluble in water hot or cold 

 and which has found many applications in the arts. 



Gelatin to be used in cooking should be nearly white and 

 should dissolve in hot water to form a practically colorless, 

 odorless solution. Inferior gelatin gives off a bad odor when 

 heated with water. 



Isinglass is a kind of collagen made from the swimming 

 bladder of certain large fishes. On heating with water it 

 yields a peculiar gelatin which dissolves completely. Com- 

 mon isinglass is largely used in clarifying beer and wine, while 

 the pure white varieties are employed in thickening soups and 



jellies. 



KERATIN. 



This is the important insoluble substance in horn, the hoofs 

 of cattle, finger nails, hair and feathers. As can be inferred 

 from the conditions under which it exists, it is not easily at- 

 tacked by water hot or cold, by weak acids or alkalies, or by 

 digestive fluids. By prolonged action of hot hydrochloric 

 acid, however, it undergoes gradual hydrolysis and cleavage 

 with formation of the usual amino acids and other products. 

 Leucine is apparently the most abundant of these products, as 

 much as 1 8 per cent of the weight of the horn shavings taken 

 having been obtained by certain investigators. Other import- 

 ant cleavage products found are tyrosine, a-aminoisovaleric 

 acid, aspartic acid, glutaminic acid, phenylalanine, a-pyrroli- 

 dine-carboxylic acid, glycocoll, etc. 



All keratin bodies contain large amounts of sulphur; some 

 of this is easily split off in the form of hydrogen sulphide. A 

 large part of the sulphur appears to be present in the complex 

 body cystin, C 6 H 12 N 2 S2O4. The xanthoproteic and Millon's 

 reagent reactions are very characteristic with horn substance. 

 The behavior of a drop of nitric acid on the finger nail is well 

 known. The reactions with alkaline lead solutions, yielding 

 lead sulphide, are easily obtained. The use of lead salts in 

 hair dyes depends on this behavior. 



