HYDROGEN 



153 



iv. Hydrogen is lighter than air. Take a full jar of the gas and hold 

 it mouth upwards below a second smaller jar held mouth downwards, as 

 shown in Fig. 103. On testing with a lighted taper observe that the gas has 

 left the lower jar and filled the upper. Many experiments, as the filling 

 of balloons or soap bubbles, may also be performed to demonstrate the 

 extremely low density of hydrogen. 



v. Hydrogen forms an explosive mixture with air. Wrap your hand 

 in a duster and with it hold the soda-water bottle (Expt. 75 i.). Take it 

 out of the water so that the water runs 

 out, and the bottle is now filled with a 

 mixture of hydrogen and air. Apply a 

 light and you will not fail to observe 

 that an explosion results. 



vi. The flame of burning hydrogen. 

 Fit a right-angled tube drawn out to a 

 point to a hydrogen generator, as shown 

 in Fig. 105. After pouring a little sul- 

 phuric acid down the thistle funnel, 

 collect a test-tube of the gas issuing 

 from the straight tube, and hold the 

 mouth of the test-tube near a flame, 

 which must be a few feet away from the 

 generator. The gas will at first go off 

 with a pop, or burn with a squeaking 

 noise, but after two or three trials it 

 will burn quietly, with a blue flame. 

 When you can cany this flame of burning 

 hydrogen to the apparatus from which the 

 gas is being produced, do so, and use it 

 to ignite the gas escaping from the pointed 

 tube. If you remember always to do 

 this, there can be no danger, for when 

 you are able to carry a flame of hydrogen 

 in a test-tube of the gas, for a distance 

 of two or three feet to the generating 

 apparatus, you may be sure that the 

 hydrogen issuing from the apparatus is 

 not mixed with air. 



Light the hydrogen in this way. 



Observe that it burns with a pale blue flame, which after a time becomes 

 yellow. This coloration is due to the glass becoming hot, and some of the 

 substances in it being burnt in the flame of hydrogen. 



vii. Hydrogen can be collected by upward displacement. Substitute 

 for the delivery tube m Expt. 75 i. a tube bent twice at right angles, as 

 shown in Fig. 104, and arrange a jar on a retort-stand in an inverted position, 

 Place a test-tube over the upright tube, and allow it to stay there for a 

 minute, and test the gas as described in Expt. 75 i. When it is pure, 

 substitute the inverted jar, and after a few minutes lift it off the stand and 

 apply a light, first taking the precaution to wrap a duster round the jar and 

 to hold it away from your face. The jar will be found to contain hydrogen. 



Preparation of hydrogen. The most convenient method of prepar- 

 ing hydrogen in the laboratory is by the action of zinc on dilute sulphuric 



FIG. 105. Arrangement for obtaining 

 a flame of hydrogen. 



