PHOSPHORUS, ARSENIC, ANTIMONY, BISMUTH 327 



of the safety valve, the steam pressure, and therefore the tempera- 

 ture, exceeds a predetermined value. They behave in the same 

 way when the water is dangerously low and the metal above the 

 water becomes too hot. 



Compounds of B smuth. When strongly heated, the metal 

 burns to form a brown trioxide Bi 2 3 . This oxide gives salts with 

 acids. Thus, with nitric acid it dissolves to form a solution of 

 bismuth nitrate : 



Bi 2 3 + 6HN0 3 -> 2Bi(N0 3 ) 3 + 3H 2 O. 



When the solution is evaporated, the nitrate appears in color- 

 less crystals. If the crystals are placed in water, a white, insolu- 

 ble basic nitrate is formed : 



Bi(NO 3 ) 3 + H 2 O ?= BiONO 3 J, + 2HNO 3 . 



This is used in medicine under the name of bismuth subnitrate, 

 fco cure stomach troubles. 



Exercises. 1. By what chemical experiments could you 

 recognize red phosphorus? 



2. Explain the fact that solid Wood's metal floats upon melted 

 Wood's metal. 



3. Why is white phosphorus always kept under water? 



