INTRODUCTION. 15 



in the former case, and to become cold or to lose 

 heat in the latter. 



35. Certain minerals have a power of attracting 

 iron, and of communicating to pieces of that me- 

 tal the power of attracting and repelling one ano- 

 ther. The power thus communicated is called 

 magnetism. Bodies having magnetism have also a 

 tendency, when left free, to point toward a cer- 

 tain quarter of the heavens. This is called pola- 

 rity. 



Magnetism is peculiar to iron, and to two other me- 

 tals, nickel and cobalt. Even in the bodies where 

 it resides naturally, it is not always of the same in- 

 tensity. 



3(5. Some bodies acquire by friction the power of 

 attracting and repelling certain other bodies. This 

 power is called electricity. 



37. The chemical action of metallic substances 

 on one another, produces a like tendency in bodies 

 which communicate with those substances accord- 

 ing to certain conditions. This power is called 

 galvanism. 



a. The four properties or powers last mentioned 

 agree in this peculiarity, that they are communi- 

 cated from one body to another by mere apposi- 

 tion without any visible admixture, or transfusion 

 of substance. In this they differ from the other pro- 

 perties 



