96 



GENERAL SCIENCE 



If a piece of iron is exposed to the weather, it rusts and 

 a reddish substance is formed. The iron is slowly oxi- 

 dized. What kind of change is this? 



Preparation of Hydrogen. - - The electrolysis of water 

 taught us that water is composed of two gases oxygen 

 and hydrogen. One of these gases we have studied so 

 that we are quite welt acquainted with its properties. 

 It now 'remains for us to study the properties of the other 

 gas in which we are so vitally interested. Hydrogen is 

 another element which as a component of water is found 



in all the tissues 

 of the body and 

 therefore is abso- 

 lutely essential to 

 life. 



Hydrogen may 

 be easily prepared 

 by bringing to- 

 gether certain acids 

 and metals. All 

 acids contain hydrogen, but the best acids for our pur- 

 pose are dilute sulphuric acid and dilute hydrochloric 

 acid. The apparatus needed for the preparation of hydro- 

 gen is shown in Figure 92. 



Experiment 27. Into a wide-mouthed pint bottle intro'duce a 

 handful of zinc. Use a two-hole rubber stopper in this bottle 

 and through one hole pass a thistle tube reaching nearly to the 

 bottom of the bottle. Through the other pass a delivery tube 

 leading to a pneumatic trough. Pour the dilute acid through the 

 thistle tube and collect the gas formed over water as in the case 

 of oxygen. The first gas evolved is always mixed with air and 

 should not be used. 



To determine whether the gas is nearly pure, collect some of 

 it in a small test tube and carry it mouth downward to a flame a 



' FIG. 92. Collecting Hydrogen. 



