CHAPTER XIV 



INTRODUCTORY. SIMPLE CHEMICAL OPERATIONS 



Physical and Chemical Changes. Matter is subject to two 

 kinds of change. Hitherto we have been chiefly concerned with 

 those which influence the properties of matter, leaving its com- 

 position unaltered. It has been seen that a body, such as a 

 piece of iron, may gradually increase in temperature, changing 

 from cold iron to hot, and, becoming hotter and hotter, may 

 change in colour, passing from a dull gray to red, and from 

 red to almost white, becoming incandescent and emitting 

 light rays. But if left to itself the iron will begin to cool, 

 passing through the same changes in the reverse order until it 

 reassumes precisely the former condition ; and in all these 

 changes the weight of the iron remains unaltered. Or, 

 again, we might take a piece of soft iron, and, having wound 

 silk-covered copper wire round it several times, pass an electric 

 current through the wire. It would be found, on examining 

 the iron, that new properties had been imparted to it, that it 

 was now able to pick up other pieces of iron, or had become 

 magnetised. If the electric current be discontinued, the new 

 power, too, disappears. Such changes as these, where the 

 substance or composition of the body remains unchanged, are 

 known as physical changes. On the other hand, if a piece of iron 

 be left exposed to damp air for some hours it becomes covered 

 with a reddish-brown powder, which the most superficial exami- 

 nation will show is a different substance from the iron with 

 which we started. There are a very large number of changes of 

 the same kind as this continually taking place around us. 

 When gunpowder explodes, we have an abundance of smoke 

 formed and a black residue left behind, and it is easy to see that 

 the smoke and deposit are quite unlike the gunpowder before 



