8 FISH ISINGLASS AND GLUE. 



set to a jelly on cooling. This fact, combined with its ability to with- 

 stand tryptic digestion, indicated the presence of collagen in the 

 original tissue, and further tests confirmed this conclusion. 



The collagen was rapidly hydrolyzed by pepsin in hydrochloric 

 acid solution. It was found to be insoluble in dilute alkalies and 

 acids but swelled in the latter on standing. 



An aqueous solution of the collagen (obtained by boiling with 

 water) was tested for gelatin as follows : The solution could not be 

 coagulated by boiling, by mineral acids, acetic acid, lead acetate, or 

 other metallic salts. It could be precipitated, however, by alcohol, 

 picric acid, tannic acid in the presence of sodium chloride, or by 

 potassium ferrocyanide in the presence of acetic acid. It gave a 

 blue-violet biuret test, but no Adamkiewicz or xanthoproteic reaction. 

 Millon's test gave only a slight precipitate with little color. These 

 tests show the presence of gelatin and the absence of other protein 

 matter. 



The presence of collagen (rough experiments showed that over 

 90 per cent of the nitrogenous matter of the swim bladder is col- 

 lagen), and the fact that it may be readily converted into gelatin 

 allow the sound of the tilefish to be put to the same use as the sounds 

 of the sturgeon, hake, and other fishes. 



MANUFACTURE AND USES OF ISINGLASS. 



Isinglass is manufactured by an exceedingl}^ simple process. The 

 industry was initiated in the United States in 1821, at Rockport, 

 Mass., cleaned hake sounds being pressed into plates. In 1834 the 

 procedure was somewhat improved, and the cleaned sounds, softened 

 to the desired consistency by soaking in water, were converted into 

 ribbon isinglass by being passed between solid rollers. The ribbons 

 were then dried. In 1848 the solid rollers were replaced by hollow 

 iron rollers, through which cold water could flow, and thus prevent 

 the ribbons from softening and sticking to the iron, as they are apt 

 to do, especially in warm weather. In 1873 a scraper was placed 

 against the rollers to remove all isinglass adhering to them. The 

 ribbons were made to the desired thickness by adjustment of the 

 space between the rollers. 



The manufacture of isinglass is best carried on through the cooler 

 months on account of the softening and putrefying effect of a slight 

 rise in temperature. The sounds received, generally, have been pre- 

 viously cleaned, perhaps scraped, de-salted, and air-dried. They 

 are usually in a hard and tough condition, so they must be first im- 

 mersed in water for several hours. Four to six hours may be re- 

 (juired for the gelatin to absorb enough water to be sufficiently pliable 

 to handle. The sounds may now be run into a cutting machine pro- 

 vided with a roller and a set of knives which chop the sounds into 



