260 ELEMENTARY GENERAL SCIENCE CHAP. 



up the oxygen required to combine with the sulphur dioxide. The 

 nitrogen peroxide is thus converted into nitric oxide, which, by com- 

 bining with oxygen from the air, is again changed to nitrogen per- 

 oxide, and all the changes are repeated again and again. In practice, 

 the sulphur dioxide, oxides of nitrogen, water in form of steam, and 

 air, are all passed into large leaden chambers on the floor of which 

 the sulphuric acid, obtained by the changes we have described, 

 collects. 



Sulphides are compounds of sulphur with some other element. 

 If sulphur is combined with a metal a metallic sulphide is the result, 

 such as zinc sulphide or blende. 



Sulphuretted Hydrogen, or hydrogen sulphide, is obtained by the 

 action of dilute acids upon certain sulphides. It is a gas with an 

 odour of rotten eggs. It burns in air with a pale blue flame, forming 

 sulphur dioxide and water. If, however, there is only a small 

 supply of air or oxygen, water is formed and the sulphur 'deposited 

 as a yellow powder. Sulphuretted hydrogen will not support com- 

 bustion. When the gas is passed into solutions of certain metallic 

 salts, precipitates consisting of metallic sulphides are thrown 

 down. 



Double Decomposition. Those changes where both reacting com- 

 pounds are converted into new products by the exchange of some 

 constituents, are known as double decompositions, thus : 



Copper , sulphuretted . rn Copper -, sulphuric 

 sulphate c hydrogen ^ lve sulphide and acid. 



Quantitative Composition of Sulphuretted Hydrogen. By heating 

 a piece of tin in sulphuretted hydrogen, contained in a bent tube 

 over mercury, the tin is made to combine with the sulphur and to 

 leave the hydrogen. If when the apparatus is cool the volume of 

 the hydrogen is measured, it is found to be the same as that of the 

 sulphuretted hydrogen taken at first. 



QUESTIONS ON CHAPTER XVII. 



1. Describe the changes which sulphur undergoes when heated. 



2. What is plastic sulphur, and how is it obtained ? How would 

 you prove it consists solely of sulphur ? 



3. Describe two methods for obtaining crystals of sulphur. What 

 differences are there in the crystals so obtained ? 



4. What happens when sulphur burns ? By what other method 

 can you obtain the product formed ? 



5. Give an account of the properties of sulphur dioxide. 



6. What are sulphites, and how are they obtained ? By what 

 characteristic reaction would you recognise a sulphite ? 



7. Describe, with a diagram, an experiment to prove that when sul- 

 phur burns the product formed weighs more than the original sulphur. 



8. Describe the appearances and properties of sulphur trioxide. 

 How is it obtained, and what is its action on water ? 



9. Give a short account of the method of preparation of sulphuric 

 acid. 



