78 Intelligence and Miscellaneous Articles. 



Engaged on optical experiments, I have recently found a method 

 which never fails and can always be easily and conveniently exe- 

 cuted. For the preparation of a brilliant and firmly adhering layer of 

 gold three solutions are prepared, which are kept separate, and when 

 used need only be mixed in definite proportions : — 



1. A solution of chloride of gold in water containing 1 grm. gold 

 in 120 cubic centims. The gold is dissolved in the smallest quantity 

 of aqua regia, the excess of acid evaporated on the sand-bath, and the 

 solution diluted 120 cubic centims. It is not necessary to heat the 

 hydrochloric acid solution until protochloride is formed ; for the pre- 

 sence of a small trace of free acid is unimportant in the formation of a 

 good mirror. The solution of gold must, however, be absolutely free 

 from such metals as are deposited in the metallic form by the redu- 

 cing-liquid, and more especially from silver. If the chloride of gold 

 contains traces of chloride of silver, most of the gold is deposited in 

 the metallic form, and the thin badly coloured mirror is soon de- 

 tached from the glass. 



2. Soda-lye of the specific gravity T06. This need not be pure ; 

 in my experiments I have used commercial soda (containing chlorine 

 and sulphuric acid) made caustic by ordinary lime, with the same 

 success as chemically pure caustic soda. 



3. The reducing-liquid. 50 grms. of sulphuric acid are mixed 

 with 40 grms. of alcohol and 35 grms. of water ; and after the addi- 

 tion of 50 grms. of finely powdered manganese, the mixture is distilled 

 at a gentle heat on the sand-bath, and the vapours passed into a flask 

 containing 50 grms. of cold water. The distillation is continued 

 until the volume of the water is doubled ; the liquid obtained, which 

 contains aldehyde along with some formic and acetic ether, is mixed 

 with 100 cubic centims. of alcohol and 10 grms. of sugar converted 

 by means of nitric acid. The change of the sugar is effected by dis- 

 solving 10 grms. of ordinary cane-sugar in 70 cubic centims. of water, 

 adding 0*5 grm. of nitric acid of the specific gravity 1*34, and boil- 

 ing for a quarter of an hour. This reducing-liquid, preserved in 

 well- corked bottles, may be used for several months with the same 

 success. 



In order to prepare a plane or a concave mirror, one part of the 

 soda-lye is mixed in a suitable vessel with four times its volume of 

 the solution of gold, and a quantity of reducing-liquid amounting to 

 3^ or at most -£$ of the whole is added. The mixture is rapidly co- 

 loured green, owing to the liquid separated ; it is immediately 

 brought in contact with the surface to be gilded, and in such a 

 manner that the gold can deposit from the top downwards. The 

 rapidity of the gilding depends on the temperature. With a mean 

 temperature of 18° C. the mirror begins to form in 30 minutes, in 

 \\ hour it is transparent with a splendid green colour, and in 2| to 

 3 hours it is so thick that it is only translucent with a dark-green 

 light. At a temperature of 50° to 60° C. the same effect takes place 

 in 20 to 25 minutes, and at a temperature of 75° even more rapidly; 

 it is not desirable to use a higher temperature, for the gold appears 

 then to adhere less firmly to the glass. Within the limits mentioned, 



