246 ELEMENTARY GENERAL SCIENCE CHAP. 



oxygen and hydrogen are bubbled after the eudiometer has been 

 filled with mercury and inverted in a trough containing the same 

 liquid. When an electric spark is passed through the mixture parts 

 of the gases combine ; and the experiment, when performed as de- 

 scribed in the chapter, shows that two volumes of hydrogen combine 

 with one volume of oxygen to form water. This process of building 

 up a compound from its elements is called synthesis. 



The opposite process of splitting a compound up into its elements 

 is called analysis. The analysis of water is effected by passing an 

 electric current through water contained in a voltameter. The 

 result of such an experiment is precisely the same as that obtained, 

 by synthesis, with the eudiometer. 



Composition of Water by Weight. Pure dry hydrogen is passed 

 over heated oxide of copper. The hydrogen combines with the 

 oxygen of the oxide, forming water, and leaves the metallic copper 

 behind. The water formed is collected and weighed. The copper 

 oxide is weighed before and after the experiment. Its loss in 

 weight tells us the amount of oxygen in the water formed. The 

 difference between the weight of the water formed and the oxygen 

 it contains tells us the weight of the hydrogen in the water. When 

 carefully performed, the experiment shows that water is formed of 

 eight-ninths its weight of oxygen and one-ninth its weight of hydrogen. 



Natural Waters generally contain dissolved materials. When the 

 amount is very large the water is called a 'mineral water. Natural 

 waters containing sulphuretted hydrogen are called sulphur -waters ; 

 those containing compounds of iron are called chalybeate waters ; 

 and those containing much carbon dioxide are spoken of as effervescent. 



Hard and Soft Waters. Those waters in which soap lathers easily 

 are said to be soft. When such is not the case the water is spoken 

 of as hard. 



Hard waters which can be softened by boiling are said to have 

 only temporary hardness. If the water cannot be thus softened, but 

 requires the addition of a chemical, its hardness is said to be 

 permanent. 



QUESTIONS ON CHAPTER XVI. 



1. What is the composition of water? Describe experiments by 

 which its composition by volume may be found. 



2. By what means is hydrogen most conveniently obtained ? What 

 are the principal properties of the gas ? 



3. Give diagrams and a short description of an experiment to 

 prove that water is produced by the combustion of hydrogen. 



4. Describe the preparation and collection of hydrogen by the 

 action of (a) sodium, (Z>) iron, on water. What other products are 

 also formed ? How would you obtain each of the products ? 

 Describe briefly their various characters. 



5. In the preparation of hydrogen from zinc and sulphuric acid, 

 what other product is formed ? How would you separate it from the 

 liquid ? Describe briefly its appearance. 



6. Describe three experiments to show the most characteristic 

 properties of hydrogen. 



