428 OPTICAL INSTRUMENTS AND SILVERING GLASS SURFACES. 
first application. The finished surface is wiped and washed in the most thorough 
manner—for the least trace of caustic soda left would destroy the mirror. The 
further processes are the same in both methods of silvering.” 
In Brashear’s process, already mentioned, the most important thing is the 
sugar solution forming the reducing agent. This greatly improves by keeping— 
a solution that has been made some months being much more effective than a 
newly-made one. I find it convenient to have always some Winchester quarts of 
it in stock ready for use. I have for convenience varied his proportions slightly 
and thus give them as I have found them work so well. For the sugar solution 
T add to a 10°/, solution of loaf-sugar, in distilled water, 10°/, of alcohol and 
4°/, of nitric acid. Solutions of 10°/, of nitrate of silver and of caustic potash 
are separately prepared, the latter one as wanted. These, with sufficient ammonia 
and a very dilute solution of nitrate of silver, and also a similar very dilute one 
of ammonia, are prepared, the latter in order to obtain that pale brown colour of 
the ammoniated solution of nitrate of silver that it is absolutely necessary to have 
before adding the reducing agent. 
Having selected a suitable dish to contain the liquid, in which the mirror can 
be placed face downwards with about $ or ? inch of liquid underneath, find on 
the basis of 1 of silver-nitrate solution to 4 of the total required liquid the 
amount of silver solution needed; to this add ammonia till the first formed pre- 
cipitate is dissolved, then add one half of this quantity of the potash solution 
(this is a variation from: Mr. Brashear’s formula that I have found works well), 
and again add ammonia till the mixed solution is quite clear, taking care to put 
in only sufficient ammonia for that purpose; then add the weak solution of nitrate 
of silver till a clear brown colour is obtained; should this become a dark brown 
some of the weak solution of ammonia will bring it to a pale brown colour, which 
must persist if the solution is left standing some time. 
The mirror, previously cleaned with nitric acid and distilled water, and sus- 
pended in the dish in distilled water of sufficient amount to make up on the 
addition of the solutions the total liquid required, is lifted out and the prepared 
solutions mixed with the distilled water and an amount of the reducing solution 
equal to about one half that of the nitrate of silver solution more or less as the 
temperature is under or over 60°; as soon as all is intimately mixed the mirror 
is immersed with one movement, beginning by dipping the edge first and lowering 
so as to prevent any air-bubbles forming under the glass. In from three to five 
minutes the silver begins to form on the mirror, the solution changing from pink 
to dark brown and black, the film thickens quickly, and in from twenty-five to 
thirty minutes sufficient silver is deposited. ‘The mirror can then be washed and 
put to soak in distilled water for a few hours, then taken out and dried and 
polished in the usual way, that is, with a soft pad af clean chamois, and going all 
over the mirror with light strokes till the bloom is all removed and a fair polish 
is obtained, finishing with a very little of the finest washed rouge, quite dry, 
lightly dusted on the pad; it is very important to well consolidate the film of 
silver by the unrouged pad before using any polishing-powder. 
It is a very good plan for any one who is not-in the habit of silvering, or to 
whom the process is strange, to try the proportions of the solutions on some 
small pieces of glass till a satisfactory proportion for the temperature (for that is 
the chief factor in varying the amount of reducing solution necessary) of the room 
in which he is working. The most important thing (after the solutions) is the 
proper cleansing of the glass, for on the proper preparation of the surface of the 
glass a very great deal depends. 
As already stated, this process is used when the glass to be silvered can be 
suspended in the liquid ; it is not suitable when we attempt to silver surfaces face 
upwards. The mud formed settles down and prevents any proper deposition of 
