98 
POrULAR SCIENCE REVIEW. 
It consists in passing chlorine gas into the molten gold by means of a clay 
pipe dipping down to the bottom of the crucible containing it. No difficulty 
is experienced from the projection of globules of gold, as might perhaps be 
expected would be the case. The greater part of the chlorine seems to be 
absorbed at once, and no violent ebullition consequently takes place. The 
chlorine converts the silver into chloride, which floats in the liquid state on 
the gold. With the apparatus used by Mr. Miller, about eight ounces of 
silver were thus separated as chloride from gold alloyed with it, and at 
nearly a imiform rate, whether the gold contained much silver or little. 
To obviate the loss of the fused chloride of silver by inflltration into the 
substance of the clay pot, the latter is to be prepared by dipping it into a 
hot saturated solution of borax in water so as to be thoroughly impregnated 
therewith, and then drying it. No absorption of chloride then occurs with 
it. The volatilization of all but a very minute proportion of chloride of 
silver is prevented by the use of borax to form a fused layer over the chloride. 
The time required for the operation to bring the gold to a fineness of, say, 
993 parts in 1000 is only a few hours, while the apparent loss of gold is very 
little more than what is known to occur in ordinary gold melting, being 2^^ 
parts in 10,000, whereas in ordinary mint melting the apparent waste is about 
two parts in 10,000. By apparent loss is meant the loss at the end of an 
operation, without taking into account the amount recoverable from ^ sweep,’ 
&c.” The slab of argentic chloride is reduced to the metallic state by 
placing it between two flat pieces of wrought iron, and immersion of the 
whole in sulphuric-acid water. But previous to this, as the cake contains a 
little gold (apparently in chemical combination), Mr. Miller recommends its 
refusion and treatment with a little carbonate of potash, which separates the 
gold and a little silver, leaving the chloride free from gold. Lastly, as 
regards the quantity of chlorine necessary, about twice the theoretical quan- 
tity only is required, half of it passing unused into the chimney. — Journal of 
the Chemical Society for December. 
The Differed Colours of Lahradorite. — A microscopical exammation of a 
number of specimens of this mineral in the collection of the Ecole Polytech- 
nique des Pays-Bas, all from the Labrador coast, has enabled M. Vogelsang 
to give an explanation of the splendid play of colours often exhibited by it. 
In the coloration of lahradorite its more or less crystalline structure plays 
an essential part, for the coloured specimens show usually a better cleavage 
than the colourless ones. The bright blue reflected by some specimens de- 
pends upon a certain crystalline state of the mineral, and is a phenomenon of 
polarisation produced by the passage of rays refracted by one lamina into 
another lamina, the planes of vibration of which do not coincide with those 
of the first, the result being a diflerence of phase and an interference of the 
luminous rays on reflection, just as with the ordinary colours of polarisation. 
Tlie golden-yellow colours proceed from a total reflection from interposed 
microlites which consist of magnetic oxide of iron, or else of diallage ; the 
red colour results from the reddish colouring of small lamella) of diallage j 
the association of these colours with the bliieish reflection accounts for the 
green and violet play of colours ; lastly, the coloured metallic reflection 
from lamina) of diallagc gives rise to the efiects of coloured aventurine. — Ar- 
chives nCcrlandaises des Sciences e.vacles ct naturellcs. 
