276 METALS. 



The proportions if zinc in its alloys with copper are givtn in the re- 

 marks on copper. Zinc is a brittle metal, but admits of being rolled into 

 sheets when heated to about 212° F. In sheets it is extensively used for 

 roofinp and other purposes, it being of more difficult corrosion, much 

 harder, and also very much lighter than lead. Its combustibility is a 

 strong objection to it as a roofing material. 



The biddery ware of the East Indies is made from an alloy of copper 

 l6oz., lead 4oz., and tin 2oz., which is melted together and then mixed 

 with 16oz. of spelter to every 3oz. of alloy. 



The white oxyd of zinc is much used for white paint, in place of 

 white lead. 



An impure oxyd of zinc called cadmia, often collects in large quan 

 tities in the flues of iron and other furnaces, derived from ores of zin 

 mixed with the ores undergoing reduction. A mass weighing 609 

 pounds was taken from a furnace at Bennington, Vt. It has been ob 

 served in the Salisbury iron furnace, and at Ancram in New Jersey, 

 where it was formerly called ancramite. 



20. CADMIUM. 



There is but a single known ore of this rare metal. It is 

 a sulphuret, and is called greenockite. It occurs in hexago- 

 nal prisms, with pyramidal terminations, of a yellow color, 

 high luster, and nearly transparent. H = 3 — 3 '5. Gr = 

 4*8 — 4*9. From Bishopton, Scotland. 



Cadmium is often associated in small quantities with zinf 

 blende and calamine. In a black fibrous blende from Przi 

 bram, Ldwe found 1*5 to 1*8 per cent. 



21. LEAD. 

 W Lead occurs rarely native ; generally in combination with 

 sulphur ; also with arsenic, tellurium, selenium, and various 

 acids. 



The ores of lead vary in specific gravity from 5*5 — 8 '2. 

 They are soft, the hardness of the species with metallic lus- 

 ter not exceeding 3, and ethers not over 4. They are easily 

 fusible before the blowpipe, (excepting plumbo-resinite) ; and 

 with carbonate of soda on charcoal, (and often alone,) mal- 

 leable lead may be obtained. The lead often passes off in 

 yellow fumes, when the mineral is heated in the outer flame, 

 or it covers the charcoal with a yellow coating. 



Where have we the first notice of the metal bismuth ? From what 

 source is it obtained for the arts 1 What is it often called in the arts 

 How is the metal obtained 1 For what is bismuth used ? How doea 

 lead occur in nature ? What is said of the tests ? 



