TIX. 251 



It occurs near Baltimore combined with iron, forming so- 

 called chromic iron. 



Chromium forms a great many oxides, like manganese. 

 That having most oxygen plays the part of an acid. Some 

 of the salts of chromic acid are valuable. Chrome yellow, 

 a well-known pigment, is a chromate of lead. Chrome or- 

 \anye is made by digesting chrome yellow with potassium 

 carbonate, the effect of which is to remove a part of the 

 chromic acid from the salt. 



The most important of the chromates, however, is the 

 potassium dichroniate, K 2 Cr 2 O 7 , a beautiful yellowish-red 

 crystalline substance. All the compounds of chromium 

 are strongly colored, and many of them are very beautiful. 

 The green color of our "greenbacks" is due to the sesqui- 

 oxide of chromium, Cr 2 O 3 , which is a very fast color, and 

 not easily attacked by acids or alkalies. 



GROUP vi. TIN. 



353. Tin. Tin is one of the most extensively useful of 

 the metals, for it is soft and malleable, and does not easily 

 tarnish. The tin-foil which you so often see shows how 

 malleable it is. Tin is used in making many of the alloys. 

 Our common tin-ware is not tin alone, but thin sheet-iron 

 covered with tin, the sheets having been dipped into the 

 melted metal. The object of the covering of tin is to pre- 

 sent a surface to the air and to liquids that is not easily 

 oxidized. For the same reason iron chains are often cov- 

 ered with tin. Pins are made of brass, and are coated with 

 an exceedingly thin covering of tin by a chemical process 

 which has been described in Part L Tin is a brilliant 

 white metal. It is quite disposed to crystallize, as may be 

 seen by a single experiment. Sponge a perfectly clean 

 piece of tin which has been slightly heated quickly over 

 with nitre-muriatic acid. After washing it in clean water 



