568 | The American Naturalist. [sede # 
believed will enable its user to measure accurately the axes of the 
isothermal ellipses, and to determine rapidly in each case whether 
apparently circular isotherms are in reality circles or slightly eccentric 
ellipses. 
A new machine for cutting and grinding thin sections of rocks and 
minerals, with stored electricity as the motive power, is described by 
G. H. Williams” 
Rock Separations.—Thallium-silver-nitrate [Tl Ag (NO,) .], 
according to Retgers,” is an excellent medium for the separation of 
mineral grains of great density. The double salt fuses at 75°, and in 
the fused condition is clear and mobile. In this condition its specific — 
gravity is 5, and this may easily be lowered by the addition of water. 
Its manipulation is simple. A small beaker containing the solidfsalt 
is placed in a water bath and heated. Upon its liquefaction the pow- 
der to be separated is added and the mixture is allowed to stand for a 
short time. As soon asa layer of clear liquid forms between the pre- 
cipitated and the floating grains the beaker is plunged into cold water. 
The salt thus consolidates rapidly. The beaker is now broken and the 
heavy grains are collected by scraping and washing. 
A new method of separating the constituents of rock powders, whose 
densities are above 2.60, has been devised by Dafert and Derby.” The 
principle involved is the suspension of small particles in gentle cur- 
rents of water. The apparatus necessary for the operation is fully 
described by the authors. Separation is not complete between pow- 
ders of nearly the same density, but there is a strong concentration of 
the heavier and the lighter ingredients in the two resulting portions of 
the separated material. 
19 Amer. Jour. Sci., Feb. 1893, p. 102. 
Neues Jahrb. f. Min., etc., 1898, Vol. I, p. 90. 
1 Proc. Roch. Acad. Sci., Vol. II, p. 122. 
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