vii PRINCIPLE OF ARCHIMEDES 103 



CHIEF POINTS OF CHAPTER VII. 



Principle of Archimedes. When a body is submerged in a liquid, 

 it loses weight equal to the weight of the liquid displaced by it. 



Expressed differently, the upthrust experienced by an object in 

 water is equal to the weight of the water displaced. 



The number of cubic centimetres in an object is also the number of 

 cubic centimetres of water displaced by such an object when it is 

 immersed in water. 



Flotation. When a rod is floated in water the weight of the 

 whole of it is equal to the weight of the liquid displaced by the 

 portion of the rod immersed. 



Applications of the Principle of Archimedes. This principle, we 

 have seen, explains why an iron ship can float and why balloons will 

 rise in the air. 



Density. Equal volumes of different substances have different 

 masses. 



Absolute density is the mass of a unit volume of a substance. 



Relative density or specific gravity is the ratio of the weight of any 

 volume of a substance to the weight of an equal volume of water at 

 temperatures other than 4 C. 



Relative density 



- _ 



weight of equal volume of water 



Relative density = 



loss of weight in water 



Relative Density or Specific Gravity Bottle. This is a small glass 

 flask holding about 50 grams of water. It is provided with a nicely- 

 fitting ground stopper, which is in the form of a tube with a very 

 small hole through it. Its use depends upon the first of the preced- 

 ing equations. 



Balancing Columns of Liquids. The densities of two liquids 

 balanced in a U-tube are in inverse proportion to the lengths of the 

 columns. Or, the density of one liquid is less than the density of 

 the other in proportion as its column is longer than the balancing 

 column of the other. 



Relative density of a liquid - column 



liquid column 



Hare's Apparatus. Two straight glass tubes are connected at the 

 top by a three-way junction, upon the unconnected end of which a 



Siece of india-rubber tubing is placed. The lower ends of the tubes 

 ip into beakers containing the liquids the relative densities of 

 which have to be determined. By applying suction to the free end 

 of the rubber tube the two liquids are "drawn up the glass tubes, 

 The heights of the liquid columns are inversely proportional to the 

 relative densities of the liquids. 



Eelative Density of Gases. (1) Find the weight, of pure dry 

 hydrogen which fills a flask under known conditions of temperature 



