Oto IDA A. BROWN. 
gravity was not more than three. Above this density the 
liquid becomes too viscous, and it was found necessary to 
use Braun’s solution (methylene iodide), which may be 
diluted with benzol (O,H,). If the methylene iodide 
becomes too opaque to use for the separation it may be 
decolourised by shaking with a few drops of a dilute solu- 
tion of sodium carbonate and a few drops of a solution of 
sodium thiosulphate (hypo), then washing with distilled 
water and drying over calcium chloride. The chief dis- 
advantage in using this liquid! is the rapid variation of 
the specific gravity with changes of temperature, which is 
0°0022 for every degree centigrade at ordinary temperatures,. 
The liquid was poured into the Brogger funnel and the 
lower tap turned on slightly to allow the liquid to fill the 
tap. This prevents air bubbling through the solution and 
disturbing equilibrium when the tap is opened later. The 
grains of rock were then introduced, the stopper put into 
position and the funnel well shaken, with the central tap. 
still open. It was found that the separation of the felspar 
from the hornblende and pyroxene was accomplished more 
effectively by holding the funnel in an oblique position, 
thus increasing the surface area of the liquid. The funnel 
was then gradually raised to the upright position, the grains 
in the lower fraction sliding along the glass towards the 
lower tap. The central tap was then closed and the lower 
fraction was run out. If the grains have not been allowed 
to stand long enough to become tightly packed, the lower 
fraction may be taken out with very little of the heavy 
liquid, so that the same solution can be used to separate 
a large quantity of rock. 
The felspar in the upper fraction was allowed to accu- 
mulate for a number of separations, till finally most of the 
solution was run out, and the felspar was washed out, 
+ Johannsen, Manual of Petrographic Methods, p, 526. 

