42 



EXPERIMENTAL GENERAL SCIENCE 



ter, lactometer, and salimeter. Since the purity of a substance 

 has a close relation to its density, these special forms of hy- 

 drometers are much used commercially in fixing the value of 

 liquids. 



44. Buoyancy. When a body sinks in a medium lighter 

 than itself, the medium in which it sinks pushes 

 upward on the body with a force exactly equal 

 to the weight of the medium displaced. This 

 force pushing upward is called buoyancy, and a 

 body which is supported in this way is said to 

 be buoyant. Cork, owing to its small density, 

 is very buoyant in water, and is much used in 

 life-preservers. It is due to the buoyancy of 

 the air, that balloons, filled with some lighter 

 gas, rise irt it, and balloons are therefore said to 

 FIG. 11. The De buoyant. It is owing to buoyancy also that 

 diving vial (small one can lift a stone in water which he would 

 bottle of water) 6 ^ n( ^ impossible to move on land. An instruct- 

 ive illustration of the principle involved may 

 be made of a good-sized bottle and a small vial. Fill the large 

 bottle with water and invert in it the small vial, taking care to 

 have just enough air in the latter to keep it afloat. When the 

 large bottle is full of water and the cork pushed in, it will com- 



FIG. 12. Spirit level. 



press the air in the vial, make more room for the water, and 

 cause it to sink. Removing the pressure will allow the air in the 

 small vial to expand and push out some of the water making 

 it lighter and causing it to rise to the surface. Submarines 



