L. MECHANICS AND ENGINEERING. 463 



nished within with a system of magnets, arranged radially. 

 The mixed ore, in its passage over a series of four of these 

 drums, has its magnetic portion gradually separated by at- 

 traction, and the part that escapes is clean blende. 16 A, 

 Oct., 1874, 539. 



DISCOVERING THE CHARACTER AND COMPOSITION OF IRON 



AND STEEL BY ETCHING. 



Of late years it has been ascertained that much informa- 

 tion can be gathered as to the chemical composition and 

 physical character of iron and steel by etching it, and watch- 

 ing the changes which take place during the operation, and 

 the appearance of the surface after it is completed. Accord- 

 ing to Professor Kick, of Prague, the best etching substance 

 consists of a mixture of equal parts of hydrochloric acid and 

 water, to which is to be added a trace of chloride of anti- 

 mony (one drop to the quart of acid). The latter substance 

 seems to render the iron less inclined to rust, so that, after 

 washing thoroughly in warm water and applying a coat of 

 varnish, the etched surface may be preserved quite clean. 

 The smooth surface to be etched is first surrounded with a 

 ridge of wax an inch high, and the water poured into the 

 disk thus formed at a temperature of 55 to 65 Fahr. The 

 action soon begins. The time required for completion is 

 from one to two hours, but the operation should continue 

 until the texture is visible. Every half hour the acid can 

 be poured off without removing the wax, the carbon rinsed 

 off, and the surface examined. When the etching is finished, 

 the wax is removed, the iron washed first with water con- 

 taining a little alkali, then clean water, brushed, dried, and 

 varnished. 3 A, Oct. 24, 1874, 523. 



NOVEL PHENOMENON DURING THE FORGING OF METALLIC 



BARS. 



At the forging of the bar of iridium-platinum-alloy for the 

 standard meter, M. Tresca states trrat there were sometimes 

 produced, upon the lateral faces of the piece under the ac- 

 tion of the hammer, streaks of light having an oblique di- 

 rection. When a bar of metal is lengthened by the blow 

 of a hammer on an anvil of the same form as the head of 

 the hammer, which blow produces above and below a sym- 



