OPTICAL INSTRUMENTS AND SILVERING GLASS SURTACES. 429 
silver ; this was a source of considerable trouble when it was required to silver 
the three-foot 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 five-foot 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 five-foot 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 five-foot 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 paraflined 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 five-foot mirror, when mounted on the 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 five-foot were for each application : 
3000 cubic centimetres of silver solution as before ammoniated as already 
described and 500 cubic centimetres of Rochelle-salt solution, with about 29,000 
cubic centimetres of 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 dried and polished. 
A very fine film of silver was deposited on a five-foot mirror, using one appli- 
cation only of 4000 cubic centimetres of silver solution and 750 cubic centimetres 
of 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 disk 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 gave a total weight of 26°5 grains of silver ona surface of 2800 
square inches, equal to a thickness of yg/59 inch, almost exactly; in actual 
weight semewhere between that of a threepenny and fourpenny piece, not a large 
amount of the 400 grammes of nitrate of silver used in depositing the film. The 
actual waste need not be very much, as the chloride of silver 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 processes all have the ammoniated solution of 
nitrate of silver, 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 flocculent 
deposit is formed, difficult of redissolution. If after the solution is cleared by 
the addition of ammonia a strong solution of silver nitrate is added to get this 
colour, this flocculent deposit occurs; but if the weak solution advised be used, 
there is not any 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, recognized by its dark-grey 
metallic lustre. This is extremely liable to explode with great violence on the 
contact of almost any thing; a few drops of water once sufficed to explode some 
in a beaker and blow it to fragments. By using moderately diluted solutions this 
