LESSONS IN PHYSICS. 23 



air, which indicates tension in the surfaces of the bubble. Pure 

 water has the highest surface tension of any ordinary liquid 

 except mercury. It is the surface tension of water that causes oil, 

 dust, or any impurities that may settle on it, to spread out over 

 the whole surface. The interesting phenomena observed when 

 trying to spread a small quantity of water over an oily surface, 

 or when a drop of oil and a drop of alcohol are made to touch 

 each other on a plate of glass, or when small quantities of water 

 and alcohol or water and ether are brought together on a plate 

 or piece of glass, are all explained by the action of surf ace tension. 

 Surface tension explains capillarity. The force of adhesion be- 

 tween glass and water is stronger than the force of cohesion 

 between the molecules of water, and the water rises along the 

 sides of the tube, making the surface of the water inside the tube 

 concave like a cup. The effect of this action is to lessen the press- 

 ure on the water in the tube, and the air pressure on the outside 

 forces the water up in the tube until the weight of the column of 

 water equals the difference between the forces of adhesion and co- 

 hesion. In the case of an oiled tube and water, or a glass tube and 

 mercury, no cohesion is manifested, and surface tension causes 

 an increased pressure inside the tube, and the liquid is depressed. 



Magnetic attraction is that form of attraction which acts 

 between a- lodestone or a magnet and iron, drawing the bodies 

 towards each other and causing them to assume some particular 

 position in reference to each other. The compass, so important 

 in navigation and in surveying, owes its value to this form of 

 attraction. 



Electrical attraction is shown in lightning, and often results 

 from chemical action. Through the action of magnetic and elec- 

 trical forces we have the telegraph, telephone, dynamo, electric 

 motor, etc., which will be more fully discussed under the subjects 

 of magnetism and electricity. 



Chemical attraction or affinity acts between the" atoms or 

 molecules of elementary bodies, binding them together into mole- 

 cules of compound bodies, whose molecules are held together by 

 the force of cohesion. 



