IN CHEMISTRY. 129 



(2). K in a similar manner, depending upon the tem- 

 perature, forms K 2 O, K 2 O 2 , and K 2 O 4 . The first, with 

 water, forms the ordinary caustic potash, HKO, of com- 

 merce. It is an alkaline base. 



(3). ZnO is the only known oxide of zinc. It forms 

 salts. 



(4). Seven compounds of S and O are known, but only 

 two are of interest the familiar anhydrides, SO 2 and 

 S0 3 . 



(5). The oxides of iron are four in number: (i) the 

 monoxide, or ferrous oxide, FeO, from which the green 

 ferrous salts are derived ; (2) the sesquioxide, or ferric 

 oxide, Fe 2 O 3 , yielding the yellow ferric salts; (3) the 

 magnetic or black oxide, Fe 3 O 4 , which does not form any 

 definite salts ; (4) ferric acid, H 2 FeO 4 , a weak acid, form- 

 ing colored salts with potassium. 



(6). Pb forms two oxides, the monoxide and the di- 

 oxide. The former is the well known litharge, which is 

 the base of the lead salts. 



(7). Cu has two oxides the cuprous (Cu 2 O) and 

 cupric (CuO), both of which form salts, thus giving rise 

 to two series, the cuprous and the cupric salts. The two 

 oxides are commonly known as the red and the black. 



(8). Phosphorus forms two oxides, phosphorous anhy- 

 dride (P 2 O 3 ) and phosphoric anhydride (P 2 O 5 ). 



39. Is charcoal lighter than H 2 O ? 



Charcoal appears at first sight to be lighter than 

 water, as a piece of it floats on the surface of this liquid ; 

 this is, however, due to the porous nature of the charcoal, 

 for if it be finely powdered it sinks to the bottom of the 

 water. ROSCOE. 



