88 JUNIOR GRADE SCIENCE 



begin to cool, passing through the same changes in the reverse order 

 until it reassumes precisely its former condition. 



Or, again, a piece of soft iron having silk-covered copper wire 

 wound round it several times, becomes altered if an electric current 

 is passed through the wire. It is found, on examining the iron, that new 

 properties have been imparted to it, that it is now able to pick up 

 other pieces of iron, or has become magnetised. If the electric current 

 is discontinued, the new power too disappears. 



Such changes as these, and those dealt with in Expt. 32 i., 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 a 

 superficial examination will show is a different substance from the iron 

 originally exposed. There is a very large number of changes of the 

 same kind as this continually taking place around us. When a lump 

 of coal burns, it is easy to recognise a great difference between tho 

 ash and smoke resulting from the burning and the original coal. So, 

 too, the burning of magnesium ribbon affords an excellent example 

 of the formation of a new substance. 



Changes which result in the formation of altogether new substances with 

 entirely new properties arc called chemical changes. 



It is with the nature and cause of changes of this kind that Chemistry 

 ia concerned. 



In the remaining pages of this book the student will be concerned 

 with chemical changes and his study of the simple examples here 

 described will serve to introduce him to the important subject of 

 Chemistry. 



