Having selected a suitable dish to contain the liquid, in which the mirror can be placed face downwards with about 1/2-3/4 in. 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 precipitate 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 silver nitrate 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 suspended in the dish in distilled water of sufficient amount to make up on addition of the solutions the total liquid required, is lifted out, and the prepared solutions are mixed with the distilled water and an amount of the reducing solution equal to about one half that of the silver nitrate solution, more or less, as the temperature is under or over 60° F.; 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 3-5 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 25-30 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 of 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 stiver by the un-rouged 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 silver; this was a source of considerable trouble when it was required to silver the 3-ft. mirror, and a pneumatic arrangement was eventually made to hold the mirror by the back, so that it could be silvered face downwards, and up to that size the silvering could be managed.

The great size of the 5-ft. mirror and its enormous weight (over half a ton without the cell) made it dangerous to suspend it, and the question of silvering became a serious one. In making experiments, in order to get rid of the mud formed in the process last mentioned, it was found that by leaving out the potash the silver was deposited from a nearly clear liquid and no mud was formed, and the first 5-ft. mirror was very successfully silvered in this manner. The solutions of silver and sugar are used in the same proportions without potash, but it is found advisable to use a stronger total mixture. For subsequent silvering of the 5-ft. mirror the Rochelle salt process has been used, and this for the deposition of the silver on a surface face up seems to be the best, using if necessary two or more applications.

In preparing a large mirror for silvering in this manner it is necessary to form it into a dish by using a band of paraffined brown paper round the edge, standing up an inch or more all round, and mounting the mirror on a swinging support, so that it can be tipped up to throw off the water or spent solutions; in the case of the 5-ft. mirror, when mounted on this machine this tipping up could be done by the same arrangement used for placing the mirror vertical for testing.

The proportions of solutions used for the 5-ft. were for each application 3000 cc. silver solution ammoniated as already described, and 500 cc. Rochelle salt solution, with about 29,000 cc. distilled water; this remained on the mirror 28 minutes; another similar application was left on for 30 minutes; after thorough washing, distilled water was left on for some hours, and the film was dried and polished.

A very fine film of silver was deposited on a 5-ft mirror, using one application only of 4000 cc. silver solution and 750 cc. Rochelle salt solution - this after one year was found to be in a very good state indeed; this was on the first mirror which, from some defect in the glass could not be made into a good mirror. The disc of glass was returned to the makers to be replaced by another. I took this opportunity of removing and collecting the whole of the silver by dissolving it in nitric acid. The assay of the deposit gives a total weight of 26.5 gr. silver on a surface of 2800 sq. in.; in actual weight somewhere between that of a threepenny and four-penny piece, not a large amount of the 400 grm. of the silver nitrate used in depositing the film. The actual waste need not be very much, as the silver chloride can be easily deposited by the addition of common salt to the spent solutions, and the silver thus recovered.

It will be seen that the various proposes all have the ammoniated solution of silver nitrate, and differ only in the reducing agent. The preparation of this solution, in order to get the pale brown colour already spoken of, demands some care. If the solution is too strong, on the addition of ammonia a very fiocculent deposit is formed, difficult of re-sol ution. If after the solution is cleared by the addition of ammonia a strong solution of silver nitrate is added to get this colour, this fiocculent deposit occurs; but if the weak solution advised be used, there is no difficulty in getting the proper colour free from any deposit. This is important. A word of caution may not be out of place concerning the production sometimes of a fulminate of silver, recognised by its dark grey metallic lustre. This is extremely liable to explode with great violence on the contact of almost anything; a few drops of water once sufficed to explode some in a beaker and blew *it to fragments. By using moderately dilute solutions this danger is obviated.