. 



POTASSIUM AND SODIUM. 213 



out the fire which the oxygen and carbon are disposed to 

 get up together. 



293. The Explosion Explained. If the gases thus evolved by the 

 burning of gunpowder could remain condensed, occupying the same space 

 that they do in the powder, there would be no explosion. But the moment 

 they are evolved they immediately expand so as to occupy a space sev- 

 eral thousand times greater than before. And it is this expansive force 

 which causes the sound of the explosion, and which constitutes the power 

 of the burning powder in propelling balls, rending rocks, etc. It is the con- 

 cussion or blow which the suddenly expanding body of gas gives to the sur- 

 rounding air that causes the detonation. Observe the difference between 

 this explosion and that which occurs when gases unite suddenly to form a 

 liquid, as in the formation of water by the explosion of oxygen and hydro- 

 gen ( 144). In the latter case there is condensation, while there is none 

 in the former. Now if the condensation were the sole cause of the detona- 

 tion, the explanation would be this : A vacuum is created by the condensa- 

 tion, and the air rushing into it from all quarters, and therefore coming to- 

 gether, produces a sound very much as clapping two hands together does. 

 But this explanation will not hold, for there is not only no evidence of col- 

 lapse at the moment of explosion, but, on the other hand, decisive evidence 

 of expansion. For example, when the gases are discharged in the gun, in 

 the experiment in 191, the cork is driven out, showing that there must be 

 expansion at the first, although eventually there is condensation. But how 

 is this expansion produced ? It must come from the fact that the water is 

 formed in the midst of the great heat which always attends the combustion 

 of oxygen and hydrogen together, and is therefore steam largely expanded, 

 to be condensed, however, at the next instant. Whether this condensation 

 has any agency in the production of the sound is uncertain. 



294. Sodium. This is a soft, light metal, somewhat re- 

 sembling potassium, and, like it, never occurs in a free state 

 in nature owing to its powerful attraction for oxygen. This 

 attraction is not quite so strong as in the case of potassium ; 

 so when sodium is thrown upon cold water it runs about 

 with a hissing sound, but does not usually set fire to the 

 hydrogen evolved. By using hot water, the sodium will 

 set the hydrogen on fire, which then burns with a bright 



