PRESSURE IN LIQUIDS 



29 



Experiment 26. Using blocks of different sorts of wood, of 

 cork, ice, Ivory soap, etc., float each upon water. In each case 

 compare the amount above the surface with that below. Try to 

 float iron, copper, lead, or rock upon mercury. 



Many heavy substances may be so shaped as to hold 

 a great amount of air, and then they may float. Many 

 vessels are now made of iron or steel ; they float because 

 they contain so much space filled with air that the 

 vessel as a whole is lighter than the same volume of 

 water. 



40. Specific Gravity of Liquids. Like solid sub- 

 stances, liquids vary much in the kind of matter of 

 which they are made, and therefore they differ in weight. 

 Hence it is desirable to know the specific 

 gravities of liquids as well, as solids. In 

 this case, also, water is used as the stand- 

 ard, the weight of the various liquid sub- 

 stances being compared with that of an 

 equal volume of water. To avoid weigh- 

 ing the liquid a simple device is commonly 

 used, called an hydrometer. An hydrome- 

 ter is a hollow tube of glass weighted at 

 one end and having a scale of specific 

 gravities marked on its stem (Fig. 14). 

 Upon being put into a liquid it sinks more 

 or less, according as the substance is light or heavy, 

 and the mark on the scale where the liquid surface 

 rests will show the specific gravity of that substance. 



Experiment 27. Shake some oil and water together in a test 

 tube. Let it stand some minutes; examine and explain. 



Put a drop of mercury into a glass of water. What happens ? 



FIG. 14 



