1864. | Physics. 159 
An improved process for silvering glass for telescopic purposes has 
been published by M. Martin.* Sle uses four liquids :—The first 
being a 10 per cent. solution of nitrate of silver; the second, liquor 
ammonie sp. gr. ‘970; the third, a 4 per cent. solution of caustic 
soda; and the fourth, a 123 per cent. solution of white sugar, to which 
he adds a 4 per cent. of nitric acid, and after 20 minutes’ ebullition 
adds 25 parts of alcohol, and water to make up the bulk to 250, The 
silvering liquid is made by mixing together twelve parts of solution 1 ; 
then eight parts of No. 2; next twenty parts of No. 3; then sixty parts 
of distilled water ; and finally, in twenty-four hours’ time, ten parts of 
No. 4. The object to be silvered is then to be immersed in, when it 
will be immediately covered with a film of reduced silver, which in ten 
or fifteen minutes’ time will be sufficiently thick for use. After having 
been well washed with distilled water and dried, the surface may be 
polished with chamois leather and rouge. 
During some researches on the compounds of mercury with the 
organic radicals, Dr. Frankland and Mx. B. Duppa discovered a sub- 
stance which they call mercuric methide. This body is a transparent 
colourless liquid, of the specific gravity of 3:069, so heavy, in fact, that 
dense lead glass floats upon its surface. It has been suggested by Mr. 
Spiller that this would be an admirable liquid for fluid prisms. At pre- 
sent the only substance suited for this purpose is bisulphide of carbon, 
which is not half the density, besides being objectionable on account of 
its offensive odour, its great volatility, and easy ignition. Mercurie 
methide is superior to bisulphide of carbon in all these respects, and 
its preparation in quantity would not be attended with any particular 
difficulty. 
A most ingenious application of some well-known facts connected 
with the reflection of light by prisms, has been brought forward by 
Mr. Henry Swan, at the meeting of the British Association. He takes 
two rectangular prisms of flint glass, placed with their widest sides in 
contact. ‘The two copule of a stereoscopic picture are placed in con- 
tact with this combination, one being at the back and the other at the 
side. Upon now viewing this arrangement with the two eyes, the 
picture at the back is seen only by one eye, whilst the side picture is 
the only one seen by the other eye, the result being that the picture 
appears projected into the centre of the block of glass, possessing as 
much apparent solidity as if it were a model cut in ivory. 
Heat.—The relation of radiant heat to aqueous vapour is being 
thoroughly investigated by Professor Tyndall.t He has found that 
pure dry air is almost perfectly transparent to heat-rays, but that, on a 
day of average humidity, the quantity of aqueous vapour diffused in 
London air produces upwards of sixty times the absorption of the air 
itself. This fact is of vast importance to meteorological science. Ten 
per cent. of the entire radiation of heat from the earth is absorbed by 
* «Comptes Rendus,’ vol. lvi. p. 1044. 
+ ‘Phil. Mag.’ vol. xxvi. p. 30. 
