39P Tin. 
the plaster mould, in order to make it lie close ; after 
which, cover it with the mercury, and, without a paper, 
(as directed for silvering plain mirrors), slide it over the 
.silvered foil ; then place a weight on it, and let it stand 
two or three days> raising it by degrees, that the mercury 
may drip off gradually. 
After this method common window glass, &c. may be 
silvered. [2 Jamison's Elements , 383. 
Observations on the polishing of Glass , and on the Amal- 
gam used for silvering Mirrors.. By B. G. Sage*. 
Having been consulted in regard to the bad effects of 
some calces or red oxyds of iron, which alter the surface 
of glass by rendering it dull and yellowish, I analysed 
these calces of iron, and found out the cause on which 
this defect depends. Red calx or oxyd of iron, called 
colcothar , is employed with water for giving the last po- 
lish to glass intended for mirrors. 
Were not the oxyd or calx of tin, commonly known 
by the name of putty, so dear, it would be far preferable 
to red calx or oxyd of iron, obtained by the decomposi- 
tion of martial vitriol, either by calcining it in a lire pro- 
per for disengaging the acid or decomposing the sulphat 
of iron by marine salt. In the latter case, the red oxyd 
or calx of iron retains a little of that salt, which is of no 
hurt in the polishing of glass : but the case is not the 
same if the colcothar or red oxyd of iron retains martial 
vitriol. This salt, when dissolved in water, is decom- 
posed, and the yellow ochre which results from ft 
penetrates the glass, forms a crust on it, and renders 
it greasy, dull, and yellowish ; a tint which is communi- 
cated to the image of the object presented to the mirror. 
Glass when smoothed and polished does not acquire 
the property of reflecting objects till it has been silvered 
*. From the Journal de Physique , Thermidor, an 12. 
