MECHANICS AND USEFUL ARTS. 91 



ammonia of commerce and add nitrate solution to it imtil a brown 

 precipitate remains undissolved. Then add more ammonia and 

 again nitrate of silver solution. Tiiis alternate addition is to be 

 carefull}^ continued until the silver solution is exhausted, when 

 some of the brown precipitate should remain in suspension. 

 Filter. Just before using, mix the Rochelle salt, and add water 

 enough to make 22 ounces. The vessel in which the silvering is 

 to be performed should be a circular dish of ordinar}^ tin plate and 

 coated with a mixture of equal parts of beeswax and resin. At 

 opposite ends of one diameter 2 narrow pieces of wood are 

 cemented to keep the face of the mirror from the bottom of the 

 vessel. 



3. The glass is slightly warmed by putting it in a tub or other 

 suitable vessel and pouring in tepid water to cover the glass; then 

 hot water is gradually stirred in. 



4. Carry the glass in the silvering vessel, into which the silver- 

 ing solution has been poured, place the whole apparatus before 

 the window, and keep up a slow rocking motion. Leave the mir- 

 ror 20 minutes or half an bour in the liquid, and wash with plenty 

 of water. 



5. When the mirror is perfectly dry, take a piece of the softest 

 buckskin, stuff it with cotton, and go gently over the whole silver 

 surface to condense the silver. You may use some of the finest 

 rouge. The best stroke is a motion in small circles ; rub an hour. 

 The thickness of the silver thus obtained is about one two-hundred- 

 thousandth of an inch. — Scientific American. 



CRYSTALLIZATION UTILIZED. 



A very curious discovery has recently been mtide In' IM. Auguste 

 Bertsch, and turned to practical account by M. Kuhlmann, the 

 celebrated chemist. Wiio is there that has not, during cold win- 

 ters, stopped to admire the beautifully symmetrical and yet fantastic 

 figures of leaves and flowers depicted on the window-panes of a 

 well-heated room, the air of which is charged with aqueous par- 

 ticles? M. Bertsch has found that Epsom salts (sulphate of mag- 

 nesia) dissolved in beer, together with a small quantity of dextrine 

 (artificial gum), and in this state applied to a pane of glass with 

 a sponge or brush, will, on crystallizing, produce the identical 

 designs above alluded to, hitherto considered peculiar to water ; 

 with this improvement, however, that the liquid may receive any 

 color whatever, at the option of the operator. The ephemeral 

 productions of frost may thus be easily perpetuated ; but M. 

 Kuhlmann, on being apprised of the fact, conceived the idea of 

 going a step further, and transferring those fairy-like creations to 

 stuffs and paper. For this purpose he first got the crystallizations 

 on sheets of iron, on which he afterwards laid one of lead. By 

 means of a powerful hydraulic press the minutest details of the 

 figures in question were durably imprinted on the soft metal, and 

 a copy of them in relief was then obtained by galvanoplastics. 

 But here another difficulty arose : in the impression of cotton 



