OPTICAL INSTRUMENTS AND SILVERING GLASS SURFACES. 427 
has worked very well. Many other processes, in which the chief variation is the 
reducing agent employed, have from time to time appeared in the various scientific 
journals—the most important being that published by Mr. J. A. Brashear in the 
“English Mechanic,” vol. 31, p. 827. This is a most excellent process and for 
ordinary work, when the glass can be put in face downwards, the best I know. 
This I give further on as I use it. 
In the ‘ Encyclopedia Britannic,” vol. 16, p. 500, two processes, hot and cold, 
are given; these, though mainly relating to the silvering of ordinary looking- 
glasses, have a bearing on the process as used for silvering mirrors. 
“In the former method there is employed a horizontal double-bottomed metallic 
table which is heated with steam from 35° to 40°C. The glass to be silvered is 
cleaned thoroughly with wet whiting, then washed with distilled water and 
prepared for the silver with a sensitizing solution of tin, which is well rinsed off 
before it is removed to the silvering table. The table being raised to the proper 
temperature the glass is laid and the silvering solution at once poured over it. 
before the heat of the table has time to dry any part of the surface of the glass, 
The solution used is prepared as follows :—In half a litre of distilled water 100 
grammes of nitrate of silver are dissolved; to this is added of liquid ammonia 
(sp. gr. 0°880) 62 giammes ; the mixture is filtered and made up to 8 litres with 
distilled water, and 7°5 grammes of tartaric acid dissolved in 30 grammes of 
water are mixed with the solution: about 2°5 litres are poured over the glass for 
each superficial metre to be silvered. The metal immediately begins to deposit 
on the glass, which is maintained at about 40° C. (104° F.), and in little more 
than half an hour a continuous coating of silver is formed. ‘The surface of silver 
is then cleaned by very carefully wiping with a very soft chamois leather and 
treated a second time with a solution like the first, but containing a double 
quantity of tartaric acid. The solution is applied in two portions, and thereafter 
the glass is once more carefully cleaned of all unattached silver and refuse and 
removed to a side room for backing up.” 
“Tn silvering by the cold process advantage is taken of the power of inverted 
sugar to reduce the nitrate of silver. This process has been adopted for the 
silvering of mirrors of astronomical telescopes, notably of Leverrier’s great 
telescope in the Paris Observatory. For ordinary mirror silvering the following 
is the process recommended by H. H. Benrath :—Two solutions are prepared, the 
first of which contains the silver salt, the second the sugar preparation. For the 
silver solution 800 grammes of nitrate of silver and 1200 grammes of nitrate of 
ammonium are dissolved in 10 litres of water and 1°3 kilos of pure caustic soda 
in 10 litres of water, and of each of these solutions 1] litre is added to 8 litres of 
water, which is allowed to rest till the sediment forms and then decanted. The 
second solution—inverted sugar—is prepared by dissolving 150 grammes of loaf- 
sugar with 15 grammes of vinegar in 0°5 litre of water, and boiling the solution 
for half an hour. After cooling it is made up with water to 4200 cubic centi- 
metres. The silvering is done on horizontal tables in a well-lighted and 
moderately heated apartment, and the glass is cleaned with scrupulous care. For 
each square centimetre of glass operated on 15 cubic centimetres of the silver 
solution above described are measured out, and from 7 to 10 per cent. of the 
solution of inverted sugar is added, both being quickly stirred together and 
poured rapidly and evenly over the glass. The reduction immediately begins and 
the solution exhibits tints passing through rose, violet, and black, till in about 
seven minutes it again becomes transparent and the deposit of metal is complete. 
This first deposit is extremely thin and allows the transmission of bluish rays. 
The exhausted solution with floating and unattached dust-like particles of silver 
is carefully wiped off, the silvered surface washed with distilled water, and again 
treated with the mixed solutions to the extent of half the quantity used in the 
