162 INTRODUCTION TO GENERAL SCIENCE 



of the water displaced. To obtain the specific gravity of iron, 

 divide the weight in air by the loss of weight. 



b. Repeat the above method, using lead, glass, and alumi- 

 num. 



c. Float the block of wood and estimate how much of it is 

 below the surface of the water. The amount below water 

 divided by the whole thickness of the block will give the 

 specific gravity of any floating body. 



117. SURFACE TENSION 



In Section 39 we learned that universal gravitation acts not 

 only between every two bodies, but also between every two 

 particles of any one body. We would expect, then, that 

 matter would contract until it became a solid mass with no 

 space between the molecules. This would happen if it were 

 not for heat, which is molecular motion. The effect of con- 

 traction, however, is visible on the surface of liquids, either 

 in films or in drops. The manifestation of this attraction 

 is called surface tension, meaning the pulling together of the 

 surface. 



On account of surface tension any liquid which is freed 

 from gravity, or is moving as fast as gravity can make it move, 

 assumes the spherical form, as in the case of raindrops or shot. 

 Shot is made by pouring melted lead from the top of a shot 

 tower and receiving the lead drops in water, which prevents 

 deformation. The circle is the figure which contains the 

 greatest area with the least perimeter, while the sphere is the 

 solid which contains the greatest amount of material with 

 the least surface. For that reason shot and raindrops assume 

 the spherical form due to their surface tension. Soap bubbles 



