158 PHYSIOLOGY FOR NURSES 



piece of steel may be magnetized and when this has been 

 done, it is found to have a positive and a negative pole, 

 just as electricity. If the rod of steel so treated is 

 broken there are two magnets, each with its two poles, 

 and this may be repeated again and again with the same 

 result. The natural magnet is an oxide of iron which 

 has the power of attracting other metals. Just as is the 

 case with the electric current, like poles repel and un- 

 like poles attract each other. 



The chief use of magnetism is in the mariner's com- 

 pass which enables the sailor night or day, cloudy or 

 fair, always to know the direction in which the ship 

 sails, and renders navigation a thing of certainty in- 

 stead of pure guess work. It is of great service, also, 

 in removing minute pieces of iron or steel from the eye 

 or from wounds inflicted by such fragments. 



CHEMISTRY 



Matter is constantly undergoing changes which are 

 either physical or chemical. If sugar is dissolved in 

 water, neither sugar nor water is changed. The sugar 

 may be extracted and the same amounts of water and 

 sugar remain. But when iron rusts on exposure to air, 

 a part of the iron has joined some of the oxygen of the 

 air and a new product has been formed. The first case 

 is purely physical and the second chemical. The first 

 is a mixture, the second a chemical combination. In the 

 first illustration neither substance has been changed in 

 character; in the second both have been altered, and, 

 while in either the process may be reversed and both 

 elements recovered, the method of recovery is different 

 the one physical and the other chemical. Chemical 

 changes are called reactions and are of two kinds, i.e., 



