EXPLOSIVE ACTIONS. 189 



sample should be collected in a test-tube like the ammonia in 

 Expt. 75 (fig. 59), and a light applied to the mouth of the test- 

 tube, after removal mouth downwards ; if on lighting this gas only 

 a very faint pop is heard (owing to the admixture of air and 

 hydrogen unavoidably taking place at the mouth of the tube when 

 uncovered to light it), the gas is fit for collection and use ; but if 

 a more vigorous small explosion occurs, the gas is still mixed with 

 air. 



Expt. 208. To illustrate the Explosive Nature of Mixed Air 

 and Hydrogen. Fill a soda-water bottle with water, place it in the 

 pneumatic trough, and partly fill it with hydrogen, so that from one- 

 third to one-fourth of the water is displaced by that gas ; now lift 

 it out of the trough altogether, so that the rest of the water runs 

 out, air entering in its place. Fling a towel over the bottle (to avoid 

 being hurt by splinters of glass, should the bottle be burst, which, 

 however, is not likely to occur with a stout soda-water bottle, 

 though it might happen with a thin flat medicine-phial), and apply 

 a light to the mouth by means of a long wax taper or spill of paper. 

 The mixture of air and hydrogen will detonate with a somewhat 

 loud report. Explosions in coal mines are produced in similar 

 fashion. An inflammable gas somewhat resembling hydrogen, 

 termed fire dainp, is naturally produced from the coal, and when 

 this becomes mixed with air in certain proportions, the mixture 

 explodes with great force when fired, thus leading to disastrous 

 accidents. Ordinary coal gas behaves in the same way, whence the 

 great danger of explosion should a pipe leak in an ordinary house, 

 or should a tap be turned on so as to allow of a large escape of gas. 



A still more vigorous explosion may be produced if pure oxygen 

 be substituted for air. Fill the soda-water bottle two-thirds full of 

 hydrogen, and then collect oxygen in it (Expt. 177), so that there 

 shall ultimately be about half as much oxygen as hydrogen present ; 

 the explosion on applying a light will now be more energetic, 

 because the explosive gases are not now diluted with the inert 

 nitrogen contained in air. 



In all these cases the cause of the explosion is simply that the 

 chemical action generates suddenly a large amount of heat, which 

 greatly expands the gaseous products of combustion (Chapter XIX.), 

 and so causes a violent motion of the air, on account of the pressure 

 produced. In just the same way, when gunpowder is fired, a large 

 quantity of hot gaseous matter is suddenly produced; if the 

 powder be in the barrel of a gun, the sudden expansion projects 

 the bullet with great force, just as a pea is shot out of a peashooter 

 by a sudden expulsion of air from the lungs into the tube behind 

 the pea. If the powder be in a cavity bored in rock or stone, as 



