176 The American Naturalist. [Febraury, 
to obtain modifications with a specific gravity greater than 3.6. His 
investigations result in the discovery that methylene-iodide will dis- 
solve iodine and iodoform, and yield a liquid with the density of 3.6. 
For separating minerals with a greater density than this, he suggests the 
use of fused silver nitrate. At 198°, this salt melts to a colorless 
liquid, with the density 4.1. A mixture of the nitrate and iodide of 
this metal give a yellow oily liquid at a temperature of 65°—70°, 
whose specific gravity (5) is greater than that of any other substance, 
that has yet been proposed for the purpose desired. The author de- 
clares that these liquids serve as convenient means for separating the 
heavier minerals of rocks, and he gives directions for manipulating 
them.——The origin of most of the siliceous sinter deposited by the 
geysers in the Yellowstone Park is stated by Mr. Weed to be due to 
asecretion of silica by algze and mosses. Waters too poor in silica to 
form deposits of this substance by cooling or evaporation, are often 
dammed back by thick jelly-like accumulations of silica, separated 
from the water by plant life, which is quite abundant in some of the 
hot springs. The geyserites and similar bodies are produced by evapo- 
ration. 

% Amer. Jour. Sci., May, 1889, p. 351. 
