322 " SCIENTIFIC RECREATIONS. 



centuries has been utilized in striking a tinder-box ; that is to say, in sepa- 

 rating, by striking a flint, small particles of iron, which ignite under the influence 

 of the heat produced by the friction. We can prepare iron in such atoms that 

 it ignites at an ordinary temperature by simple contact with the air. To 

 bring it to this state of extreme tenuity, we reduce its oxalate by hydrogen. 

 We prepare an apparatus for hydrogen as shown in fig. 310, and the gas pro- 

 duced at A is passed through a desiccative tube, B, and finally reaches a glass 

 receptacle, C, in which some oxalate of iron is placed. The latter salt, under 

 the combined influence of hydrogen and heat, is reduced to metallic iron, 

 which assumes the appearance of a fine black powder. When the experi- 

 ment is completed the glass vessel is closed, and the iron, thus protected from 

 contact with the air, can be preserved indefinitely ; but if it is exposed to the 



Fig. 311. Pyrophoric iron. 



air by breaking off the end of the receptacle (fig. 311), it ignites immedi- 

 ately, producing a shower of fire of very beautiful effect. Iron thus pre- 

 pared is known under the name of pyrophoric iron. Iron is acted upon in a 

 very powerful manner by most acids. If some nitric acid is poured on iron 

 nails, a stream of red, nitrous vapour is let loose, and the oxidised iron is 

 dissolved in the liquid to the condition of nitrate of iron. This experiment 

 is very easy to perform, and it gives an idea of the energy of certain chemical 

 actions. We have endeavoured to represent its appearance in fig. 312. 

 Fuming nitric acid does not act on iron, and prevents it being attacked by 

 ordinary nitric acid. This property has given rise to a very remarkable 

 experiment on passive iron. It consists in placing some nails in a glass, 

 into which some fuming nitric acid is poured, which produces no result ; 

 the fuming acid is then taken out, and is replaced by ordinary nitric acid, 

 which no longer acts on the iron rendered passive by the smoking acid. After 



