262                                  Pearls.
	gold, minute quantities of the precious metal, probably in the state of a chloride, being held in solution in the stream, and received into the system of the mollusc. "I notice," said Dr. Lewis, "that colours are most brilliant in regions where gold may be suspected. In the lake regions of the western States, minerals are abundant, and the conditions are not incompatible with the supposition that gold is sparingly disseminated among them, in quantities too small perhaps, to be available. . . . Of two streams producing identically the same species, one will give a large proportion of white nacres, and the other will present coloured nacres ; and usually we also notice another phenomenon — a greater brilliancy of nacre where rich colours abound. In this case I have my private opinion that gold produces its peculiar tonic effect, for tonic it is, under certain circumstances, by increasing the secretion." Dr. Isaac Lea, the great authority on the genus Unio, was not disposed to attribute the colour to any auriferous compound ; and remarked that Dr. Draper had calcined purple shells, and could not detect in the white calx any metallic substance. According to Dr. Lea, the colours of many of the American Unionidee with purple, pink, and salmon-coloured nacre, are probably due to the presence of some organic compound, such as is supposed to