378 ANNUAL OF SCIENTIFIC DISCOVERY. 



complanatus and of U. Hgamentinus, when colored, are such as result from the 

 presence of gold in a state of atomic division and dissemination in a semi- 

 opaque body. I think nitro-muriatic acid, with a minute trace of gold in it, 

 if applied to shells, will produce colors, but I never have satisfactorily demon- 

 strated this. My observations 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 

 sparingly disseminated among them, in quantities too small, perhaps, to be 

 available, but no doubt it is there. 



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

 Uniones are 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 sup- 

 posed. 



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 secretions. 



To have gold in a shell, it is not necessary that it should be an oxide. It 

 is only necessary it should have been received into the circulation of the 

 animal in solution, as chloride, or some other possible soluble form that 

 chemistry has not brought to light; and, when once in the circulation, it 

 may be eliminated, by being deprived of its solving principle, and excreted 

 or secreted with the other solid matter that enters into the formation of the 

 shell. The stannate of gold, or purple of Cassius, may be wholly deprived 

 of the tin associated with it, yet retain its purple color, and its condition of 

 atomic division, if so you are pleased to call it. But I only offer this as sug- 

 gestive of something for those interested to follow further. I am not enough 

 of a chemist to develop any facts out of a suspicion of this kind. 



Mr. Lea remarked, after reading the above extracts, that the purple, pink, 

 and salmon colors of many of our American Unionidce had had his attention 

 from the period of his first studying this beautiful and interesting family, 

 more than thirty years since. Without having experimented himself upon 

 them, he was aware that no chemist had been able to detect the presence of 

 a metal or other elementary body. He therefore thought it likely to be 

 caused by the presence of some organic body which had not yet been 

 detected; such is supposed by chemists to be the case with the colored fluates 

 of lime, colored quartz, etc. What Dr. Lewis states as regards the colors 

 being more frequent and more intense in the waters of Michigan, and in the 

 streams leading into the northern great lakes from the southern side, is very 

 true. The Unio rectus is usually white in the Ohio, though sometimes tinted 

 with purple and salmon-color, while in the more northern waters it is usually 

 of a fine,' rich purple or salmon. Two specimens from the upper Mississippi, 



