732 Transactions of the American Institute. 



June 11th, 1868. 



Professor S. D. Tillman in tlie chair ; Charles E. Emery, Secretary. 



Tlie cliairman opened the proceedings by reading the following 

 suminarj- of recent scientific investigations : 



The Gold Bearing Rocks. 



Sir Roderick Murchison, in the new edition of his " Sihiria," has 

 modified the views first put forth by him as to the distribution of gold 

 in the earth's crust. His most recent conclusions are : 



1. That looking to the world at large, the auriferous veinstones in 

 the lower silurian rocks contain the greatest quantity of gold. 



2. That where certain igneous eruptions penetrated the secondary 

 deposits, the latter have been rendered auriferous for a limited dis- 

 tance only beyond the junction of the two rocks. 



3. That the general axiom before insisted upon remains, that all 

 secondary and tertiary deposits (except the auriferous detritus in 

 the latter) not so specially afiected never contain gold. 



4. That as no unaltered purely aqueous sediment ever contains 

 gold, the argument in favor of the igneous origin of that metal is 

 prodigiously strengthened, or, in other words, that the granites and 

 diorites have been the chief gold producers, and that the auriferous 

 quartz bands in the Paloaozoic rocks are also the result of heat and 

 chemical agenc3^ 



PUOSPHOKESCEXT PhOTOGEAPHS. 



The Berlin Archiv, for April, gives the following method for 

 making photographs which are invisible in the light, but are lumi- 

 nous in the dark: Sulphate of baryta (heavy spar) is finely pulverized, 

 mixed with gum tragacanth and heated in a closed crucible. Instead 

 of heavy spar, a carbonate of baryta, strontia, or lime, with half its 

 weight of sulphur, may be used. The pulverized product should be 

 kept in a closed bottle. A sheet of albuminized paper is moistened 

 with wet blotting paper ; when sticky the pow^dered substance is dis- 

 tributed equally over it by a tuft of cotton and then dried in the 

 dark. It is printed under a glass positive. Only a few seconds are 

 required when baryta is used ; with magnesia a little longer time is 

 needed. In the dark, the light portions show the peculiar phospho- 

 rescent luminosity, the dark portions being distinguished by a want 

 of phospliorescence. The picture "will, however, slowly disappear. 



