Coloured Pearls. 263 



produce the colour of certain kinds of fluor-spar and 

 quartz. It is a curious fact that the Unio rectus is 

 usually white in the Ohio river, while in the more 

 northern waters it is generally of a fine rich purple 

 or salmon colour. Again, the Unio ligamentinus has 

 probably never been found, either pink or purple in 

 the Ohio, while at Grand Rapids, in Michigan, the 

 pink and salmon shells are quite common. In like 

 manner the Margaritana margaritifera — or common 

 Pearl-mussel — has usually a fine purple nacre in the 

 Columbia river and its tributaries, while it is almost 

 universally white in the rivers of Pennsylvania, 

 Connecticut, and Massachusetts, as is also the case 

 in Northern Europe. Instances like those seem to 

 prove that the colour is caused by certain foreign 

 substances, which are present in some streams and 

 not in others. 



It seems probable that the gold theory of Dr. 

 Lewis may explain the origin of the purple and 

 violet colours of certain Pearls, inasmuch as it is 

 well known that such tints are readily yielded by 

 salts of gold : thus the magnificent dye termed 

 " purple of Cassius " is a compound of the chlo- 

 rides of gold and tin. According to the American 

 conchologist, Mr. Tryon, jun., the Pearls found 



