182 CHEMISTRY. 



Hydro-sodium sulphate. Sodium sulphate. Sulphuric acid. 



2NaHS0 4 = Na a SO 4 + H a SO 4 



A large number of the sulphates are very soluble ; the 

 sulphates of the alkaline earths are notable exceptions. 

 Sulphates are often formed in nature from the sulphides by 

 the latter taking up oxygen from the air. 



248. Sulphuretted Hydrogen, H 2 S. This may best be pre- 

 pared by acting on ferrous sulphide with hydrochloric or 

 sulphuric acids ; heating is not necessary. 



Ferrous sulphide. Sulphuric acid. Ferrous sulphate. 



FeS + H a S0 4 FeS0 4 + H a S 



The apparatus used is shown in Fig. 80. The gas which 

 comes over is colorless, and has a very strong odor of rotten 

 eggs. It is produced in the decomposition not only of eggs, 

 but of other animal substances, and also of such vegetable 



substances as contain 

 s u 1 p h u r, as pease, 

 beans, onions, etc. 

 When at all concen- 

 trated this gas has a 

 very decided effect 

 upon various metals 

 and their salts. Thus 

 it blackens white paint 

 because it attacks the 



_ white-lead in it, form- 



Fig. so. ing a sulphide of lead. 



Silver or copper vessels exposed to it become dark from the 

 formation of sulphurets of these metals. There is some lit- 

 tle of this gas very generally in the atmosphere, and hence 

 silver articles become slowly tarnished. When much con- 

 centrated it is a very deadly gas, and it is this which oc- 

 casionally destroys the lives of men engaged in cleaning 



