FISH ISINGLASS AND GLUE. 5 



and sink or to maintain its position at a certain water level. In a 

 few fishes it may take on the functions of the limg of higher verte- 

 brates and may be considered to be the homolog of that organ. 



The size of the air bladder varies to a great extent, being very 

 small in some species, whereas in the sturgeon, hake, catfish, and 

 carp it is highly developed. In some fishes the sound is practically 

 loose in the abdominal cavity, while in others it clings closely to the 

 backbone, the intestines, and the abdominal wall. The sound is 

 made up of several tunics of which the inner laj^er is thin, often with 

 a silvery luster, containing crystalline substances, sometimes covered 

 with a pavement epithelium. The adjacent layer is thick and with 

 a fibrous structure; it is the collagen contained in this layer which 

 is the source of commercial isinglass. 



Isinglass (probably a corruption of the Dutch huisenblas, German 

 hausenblase, literally sturgeon's bladder) has for centuries been 

 manufactured and exported from Eussia. Several varieties of the 

 sturgeon {Acipenser huso or beluga, A. ruthenus or sterlet, A. sturio 

 or common sturgeon, A. stcUatus or starred sturgeon), the catfish 

 {Silurus gJanis), and the carp {('yprinus carpio)^ flourishing in the 

 Volga and other rivers, in the Caspian and Black Seas, and in the 

 Arctic Ocean, yield the well-known Russian isinglass. 



Russian isinglass is generally brought to the great fair at Nijni 

 Novgorod and from there finds its way, through the agency of Petro- 

 grad traders, to London and elsewhere. Other sources of supply 

 than Russia are Brazil, Venezuela, the East and West Indies, Penang, 

 Bombay, Manila, Nova Scotia, Newfoundland, and the United States. 

 Russian isinglass is known in commerce as staple isinglass, and is 

 sold as long and short staple, according to size. 



Leaf isinglass (Astrakhan leaf, Saliansky leaf, Samovy leaf, 

 etc.) is prepared by soaking the sounds in warm water, whereby dirt 

 and mucous membrane are removed. The sounds are then opened 

 and dried by exposing the inner membrane to the air; the dried 

 sounds may be further treated by pounding and rubbing until the 

 outer membrane is detached and separated from the purer, inner 

 layer. Book isinglass is prepared in a similar manner, but the 

 sounds are folded and covered with a damp cloth. Trimmings from 

 the leaf or book are pressed into cakes or tablets or rolled into rib- 

 bons and sold as lower-grade isinglass. The trimmings from the 

 sounds and other parts of the fish are often boiled in water until 

 the gelatin dissolves and the filtered solution is evaporated to dry- 

 ness. There is also cake isinglass, so called from its shape, although 

 sometimes it is made in a globular form. 



Long staple and book isinglass are the best varieties, a 2 per cent 

 solution in hot water setting to a jelly when cold, and yielding only 

 0.05 per cent insoluble matter. Cake isinglass is dark colored and 



