4 FISH ISINGLASS AND GLUE. 



CoUagens are to a certain extent differentiated by the ease with 

 which they are converted into gelatin. Thus the coUagenous car- 

 tilage of the trachea is transformed at 100° C. into gelatin, whereas 

 ear cartilage requires a temperature of 110° ; the collagen of air 

 bladders forms gelatin at room temperatures. In general, the for- 

 mation of gelatin takes place most readily with fishes and amphibia, 

 more slowly with birds, and very slowly with old animals. The 

 presence of salts, for example, of sodium chloride in a concentration 

 of 10 per cent retards the transformation. 



Gelatin (French gelatine^ Latin gelata^ that which is congealed) 

 is a colorless, amorphous substance which is transparent when in 

 thin sheets. It does not dissolve in cold water, but swells in this 

 medium. If treated with warm water, it dissolves to a sticky liquid 

 which, if sufficiently concentrated, sets to a jelly on cooling. If 

 gelatin is boiled with water for several hours (or simply digested 

 with water for two days at a temperature of 37° C), it is converted 

 into a nongelatinizing form ; further boiling transforms it into pro- 

 teoses, peptones, and finally into acids, among which glycocoll is 

 present in a characteristically large amount. This fact should be 

 borne in mind in a study of the commercial uses of gelatin, since 

 prolonged boiling, especially in the presence of acids, changes it 

 chemically and physically (the gelatinizing) and correspondingly 

 the adhesive power is destroyed. 



As a food, gelatin has little nutritive value, and should not be 

 substituted for other proteins of the normal diet since not all of its 

 nitrogen is in a form which can be utilized by the organism. 



Gelatin, obtained as described above, should not be confused with 

 the products derived from alg?e and seaweeds of different varieties, 

 especially those of the East Indies, China, and Japan. Thus the 

 gelatinizing substances obtained from bird's nests, prized as a deli- 

 cacy by the Chinese, and Bengal isinglass, or agar, yield carbohy- 

 drates in large amount and have no relation chemically to true 

 gelatin. 



FISH SOUNDS. 



While the principal supply of gelatin is to-day obtained from the 

 refuse of animal bones, hides, and hoofs in the slaughter and pack- 

 ing houses, the peculiar properties of the gelatin derived from fish 

 sounds, called isinglass in the trade, makes this product of consid- 

 erable commercial importance. 



The fish sound (air bladder, or swim bladder) is a hollow sac, con- 

 taining gas (oxygen, carbon dioxide, and nitrogen), situated in the 

 abdominal cavity below the vertebral column. Its principal func- 

 tion is probably mechanical. Since it is compressible, it serves to 

 regulate the specific gravity of the fish, enabling the latter to rise 



