MECHANICS AND USEFUL AP.TS. 75 



rainbow, can be imparted to zinc surfaces, by a simple chemical 

 application continued a length of time proper for the desired 

 color. It is necessary that the metal be pure, and especially free 

 from lead. It is therefore to be rubbed with siliceous sand 

 moistened with hydrochloric acid, then dipped in water and 

 rubbed vigorously with blotting paper. The zinc is then im- 

 mersed in a solution of 3 parts by weight of dry tartrate of copper 

 in 4 parts caustic soda, with 48 parts distilled water, the whole at 

 a temperature of about 50 Fall. The colors will appear suc- 

 cessively, in the prismatic order, according to the period of im- 

 mersion. In two minutes, the violet will appear ; in three, dark 

 blue ; in four and a half, a golden yellow ; in eight and a half, a 

 red purple. Intermediate terms give intermediate tints. When 

 colored, the zinc is well washed with water, and for greater per- 

 manence of color may be varnished. 



NEW AGENT FOR AMALGAMATING GOLD. 



The value of sodium amalgam has been thoroughly tested in 

 the Pacific States of America, and better results have been ob- 

 tained with it there than in any other mining district, yet it is 

 now found that it can be entirely dispensed with by the substitu- 

 tion of a well-known and much cheaper chemical compound, 

 cyanide of potassium. It has always been considered that sodium 

 amalgam owed its value to its power to attack and decompose 

 the oxides of many of the metals, and it is now found that cyanide 

 of potassium possesses the same property. It has been success- 

 fully used both on copper plates and in the pans. The plates are 

 first cleaned with sand and nitric acid, and well washed in cold 

 water ; the surface is then swabbed over with the cyanide solution, 

 and the mercury applied immediately, and rubbed on well. The 

 plates will thus get a highly sensitive coating of mercury, which 

 will seize upon the gold as it passes over them. In the pans the 

 cyanide solution is applied with each charge of mercury, the pro- 

 portion being varied to suit the ore operated upon. Mining 

 Journal. 



TRANSPARENCY OF METALS. 



Metals have generally been considered as opaque bodies, not 

 permitting the passage of light through their substance. It is, 

 however, very easy to show, b} 7 the use of an extremely thin film, 

 as of gold or silver deposited upon glass, that light passes quite 

 freoly through it, and this property has latterly been turned to 

 very good advantage. One of the earliest applications was as a 

 substitute for the ordinary soot-blackened or colored glass, used 

 in observing the sun during an eclipse, or at other times ; and the 

 silvering of the objective glass of the great telescope of the Paris 

 Observatory has permitted an investigation of the sun's disk such 

 as could not otherwise be prosecuted. Viewed through a lens, or 

 even a plane glass thus silvered, the sun appears of a soft, bluish 

 color, very sharply defined against a black background formed 

 of the sky. All the peculiarities of the solar image, the different 



