158 BRITISH FRESH-WATER FISHES 



by a thread. Many bleak will gather round it, and you 

 shall reap the reward of foresight by casting your gentle, 

 your bluebottle, or your artificial fly among them. 



There remains to be noticed the chief economic use which 

 has been found for the bleak. The silvery lustre of its 

 scales is chiefly the result of a substance called guanin, 

 which exists, more or less, in the scales of most fish, but is 

 more abundant and of better quality in those of the bleak 

 than of any other British fresh-water fish. Guanin is 

 named from its presence in guano, the fossil or accumulated 

 excrement of sea-birds ; but inasmuch as it is not found in 

 their recent excrement, it is believed to be formed by the 

 oxydising action of the atmosphere upon uric acid. Very 

 long ago the secret was discovered of applying the glittering 

 substance on the scales of the bleak to the manufacture of 

 artificial pearls. The Chinese, it is said, were the inventors 

 of the process ; but the industry became firmly established 

 in France more than two hundred years ago, when a great 

 demand ensued, and was maintained till Yarrell's day 

 (1784-1856), for Thames bleak and whitebait, which were 

 manufactured into what was termed in the trade essence 

 d? Orient. 



The process is described as follows : The scales on the 

 abdomen having been scraped, washed, and triturated in 

 water, the iridescent pigment falls to the bottom of the vessel. 

 It is said to be either phosphate of lime or phosphate of 

 magnesia, containing guanin. The sediment is then placed 

 in liquid ammonia and preserved for use, as essence d'Orient, 

 the best being yielded by the whitebait * and valued as 

 high as five guineas an ounce ; the scales of the bleak being 

 next highest in repute. Roach and dace yield a more plentiful, 

 but inferior kind of pigment, not so lustrous and tinged 

 with yellow. 



* Formerly believed to be an adult fish, but now proved to be the 

 young of the herring and the sprat. 



