270 ANNUAL OF SCIENTIFIC DISCOVERY. 



after about a quarter of an hour it reflects objects, and the reduction may be 

 considered as complete, when the bath is covered with a reflecting pellicle of 

 metallic silver. In this operation all the silver is precipitated in the metallic 

 state, and only a very small fraction of the reduced metal is fixed on the 

 object which it has been proposed to plate. According to a very exact 

 determination, the silver thus fixed on a glass surface of 226 square centimetres 

 is 49 milligrammes. Consequently, to plate a square metre of glass surface, 

 we should only require 2'210 grs, of silver, which is but a trifle. 280 cubic 

 centimetres of liquid were required to plate those 226 square centimetres of 

 superficies; now, 280 c.c. of liquid contain 1170 milligrammes of silver, conse- 

 quently, in the interior of the vessel and the sides, were deposited 1170 49 = 

 1121 milligrammes of silver, which were collected and again converted into 

 nitrate. When the operation is concluded, the object plated is removed, 

 washed with hot distilled water, and dried in a warm place. Care must be 

 taken not to touch the plated surface during these operations, or else the water 

 will get in under the metallic layer, and cause it to fall off. After desiccation 

 this layer adheres so strongly that it can scarcely be removed with the nail. 

 If the plating has been performed on the surface of the, bath, the layer will be 

 of equal thickness all over, and with proper polishing, will have every quality 

 of a silvered mirror. To attain unfailing success, several precautions are 

 necessary : in the first place, the bottom of the vessel should be all over of an 

 equal distance from the surface of the future mirror, otherwise some parts of 

 this surface will be covered with a thinner plate of silver than others, and 

 will consequently appear duller ; for they will not reflect the light so well as 

 the parts which are more thickly covered. Another important point is that 

 the surface to be silvered should be perfectly clean ; glasses simply wiped 

 with a cloth will take a layer full of patches, on which will be plainly visible 

 the streaks left by the cloth. Then the surface to be silvered must be wetted 

 all over; the smallest bubble of air will produce a hollow in the layer. 

 Before putting the glass on the bath it is better to wet it completely ; the 

 liquid used for tin's should be alcohol, in preference to water, as it drives off 

 the air better. The glass would be plated as well at the bottom of the bath 

 as at the top, only that in that case the whole of the silver would accumulate 

 on the object, and the mirror would necessarily be so much the clearer. 

 When the mirror is dry, the layer is covered with a colorless varnish composed 

 of an alcoholic solution of gum dammar ; it is then framed. 



Gilding. To gild glass permanently, heat is essential ; gilding without heat 

 is of a beautiful color and very brilliant, but it does not adhere strongly, and 

 will strip off with washing. The gilding bath is prepared by means of a solu- 

 tion of gold in aqua regia, a solution to which is added 292 milligrammes of 

 chloride of sodium per gramme of gold ; it is evaporated to dryness, and 

 heated until all the free acid is disengaged ; after which the double salt is 

 dissolved in water in sufficient quantity to make 100 c.c. of liquid contain 

 1000 milligrammes of gold. With this solution two others are prepared, one 

 of which is composed of solution of gold, 50 c.c. ; soda ley (density 1'035) ) 

 20 c.c. ; water, 300 c.c. It is boiled in a flask until reduced to 250 c.c. The 

 second solution is prepared with solution of gold, 50 c.c. ; ley, 20 c.c. ; water 



