[Communicated March 20th, I860.] 



Mr. Lea read extracts from letters of Dr. Lewis, of Mohawk, New York, on 

 the subject of the coloring matter of the nacre of the genus Unio, and exhibited 

 some fine specimens to illustrate the subject. The following extracts will folly 

 convey Dr. Lewis's ideas on this subject which has much interest with the 

 naturalist. 



" I hinted something about Untunes being colored with an oxide or salt of 

 gold. My reasons for this are derived from observing some singular phenomena 

 in colors on submitting shells to the action of chloride of gold, and then bring- 

 ing them in contact with tin. Whether a stannate of gold formed and precipi- 

 tated on the shells or not, I cannot say, but the colors were very much intensi- 

 fied. It is to be remarked that the colors of such shells as Unio complanatus 

 and of U. ligamentous, when colored, are such as result from the presence of gold 

 in a state of atomic division and dissemination in a setni-opake body. 1 think 

 nitro-muriatic acid with a niinuta trace of gold in it, if applied to shells, will 

 produce colors, but I never have satisfactorily demonstrated this. My observa- 

 tions are derived from having once used acid in which was a small quantity of 

 gold, too small to be reclaimed." 



■■ I notice that colors are most brilliant in regions where gold may be sus- 

 pected. In the Lake regions of the Western States, minerals are abundant, 

 and the conditions are not incompatible with the supposition that gold is spar- 

 ingly disseminated among them, in quantities too small perhaps to be available, 

 /"</ no doubt it is there." 



■As regards colors in the nacre of Uniones, you are correct in saying that 

 Unionesare colored where there is no gold. But there are some species that are 

 not colored unless you find them in some particular localities. If that is taken 

 into consideration we shall, perhaps, be more ready to accept the gold theory. 

 Modern investigations show that gold exists in soils that, until they were rigidly 

 tested, were not suspected to contain it. In fact I am disposed to believe that 

 gold is more universally disseminated than is generally supposed." 



" But, the question is one I take no particular interest in, except that it pre- 

 sents itself incidentally. I know one fact that you also know. That of two 

 streams producing identically the same species, one will give a large propor- 

 tion of white nacres, and the other will present colored nacres, and usually we 

 also notice another phenomenon — a greater brilliancy of nacre where rich colors 

 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 secre- 

 tions." 



