PERCUSSION. 97 



A. As the particles of the nail are 

 compressed by the hammer, it cannot 

 contain so much heat as it did before ; so 

 some of it flies out (as water flows from 

 a sponge when it is squeezed). 



Q. Why does STRIKING a FLINT against a 

 piece of STEEL produce a SPARK? 



A. The blow condenses those parts of 

 the flint and steel which strike together, 

 and squeezes out their latent heat. 



Q. How does this development of HEAT 

 produce a SPARK ? 



A. A very small fragment (either of 

 the steel or flint) is knocked off red-hot, 

 and sets fire to the tinder on which it falls. 



Q. Why is it needful to keep BLOWING the 

 TINDER with the breath ? 



A. Because blowing the tinder, drives 

 the oxygen of the air towards it. 



Q. Where does the OXYGEN of the air COME 

 FROM, which is blown to the lighted tinder ? 



A. The air itself is composed of two 

 gases f nitrogen and oxyyen} mixed 

 together. 



(Every 5tf>s. of common air contain 41bs. of nitrogen, 

 and lib. of oxygen.) 



K 



