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. 
