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 

 UniOy 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 Unionidce with purple, pink, and salmon- 

 coloured nacre, are probably due to the presence 

 of some organic compound, such as is supposed to 



