90 ANNUAL OF SCIENTIFIC DISCOVERY. 



ish-gi'een, while a beautiful violet is obtained by immersing it for 

 a single instant in a solution of chloride of antimony and rubbing it 

 with a stick covered with cotton. The temperature of the brass at 

 the time the operation is in progress had a great influence upon 

 the beauty and delicacy of the tint ; in the last instance it should 

 be heated to a degree so as just to be tolerable to the touch. A 

 moire appearance, vastly superior to that usually seen, is produced 

 by boiling the object in a solution of sulphate of copper. Accord- 

 ing to the proportions observed between the zinc and the copper, 

 in the composition of the alloy, so will the tints obtained var}''. In 

 many instances it requires the employment of a slight degree of 

 friction, with a resinous or waxy varnish, to bring out the wavy 

 appearance characteristic of moire, which is also singularly en- 

 hanced by dropping a few iron rails into the bath. There are two 

 methods of procuring a black lacker upon the surface of brass. 

 The one, which is that usually employed for optical and scientific 

 instruments, consists in first polishing the object with tripoli, then 

 washing it with a mixture composed of 1 part of nitrate of tin and 

 2 parts of chloride of gold, and after allowing this wash to remain 

 for nearly a quarter of an hour, wiping it off with a linen cloth. 

 An excess of acid increases the intensity of the tint. In the other 

 method, copper turnings are dissolved in nitric acid until the acid 

 is saturated ; the objects are immersed in the solution, cleaned, and 

 subsequently heated moderately over a charcoal fire. This process 

 must be repeated in order to produce a black color, as the first 

 trial only gives a deep green, and the finishing touch is to polish 

 with olive oil. Much pains is taken abroad to give brass objects 

 ** an English look." For which purpose they are first heated to 

 redness, and then dipped in a weak solution of sulphuric p^cid. 

 Afterward they are immersed in dilute nitric acid, thoroughly 

 washed in water, and dried in sawdust. To effect a uniformity 

 in the color, they are plunged into a bath consisting of 2 parts of 

 nitric acid and 1 part of rain-water, where they are suffered to re- 

 main for several minutes. Should the color not be free from spots 

 and patches, the operations must be repeated until the desired 

 effect is produced. — The Engineer. 



SILVERING GLASS MIRRORS. 



The process we propose to describe has for its author Prof 

 Henry Draper, of this city, and may be divided into 5 operations, 

 namely, the cleaning of the glass, the preparation of the silvering 

 solution, the warming of the glass, the process of silvering, and 

 the polishing. The description is for a 15^-inch mirror. 



1. Rub the glass plate thoroughly with aquafortis, and then 

 wash it with plenty of water and set it on edge on filtering paper 

 to dry; then cover it with a mixture of alcohol and prepared chalk 

 and rub it in succession with cotton flannel. 



2. Dissolve 560 grains of Rochelle salt (tartrate of soda and 

 potassa) in 2 or 3 ounces of water, and filter ; dissolve 800 grains of 

 nitrate of silver in 4 ounces of water. Take an ounce of strong 



