34 CHEMICAL PHYSICS. 



first in air, then in a liquid of known specific gravity, but having no solvent 

 action on the substance. The weight of the solid in air divided by the weight 

 of the liquid displaced and the quotient then multiplied by the specific gravity 

 of the liquid employed, gives the specific weight of the substance examined. 



The specific gravity of a solid lighter than water is determined by weighing 

 it first in air, then in water attached to some heavy substance, the weight of 

 which in water has been ascertained. The two substances combined will weigh 

 less in water than the heavy solid alone. The difference in weight between the 

 two weighings in water added to the weight of the light substance in air gives 

 the weight of water displaced, and this sum, divided into the weight of the 

 solid in air, gives its specific gravity. 



The method of finding the specific gravity of a solid by weighing in a liquid 

 depends on the Principle of Archimedes, which says : A body immersed in a liquid 

 loses a part of its weight equal to the weight of the displaced liquid. 



The specific gravity of liquids or gases is determined by weighing in suitable 

 glass flasks equal volumes of the standard substance and the body to be examined. 

 The weight of the latter divided by the weight of the standard substance gives 

 the specific gravity. 



Small glass flasks suitably arranged for taking the specific gravity of liquids 

 are known as pycnometers. 



A second method by which the specific gravity of liquids may be determined 

 is by means of the instruments known as hydrometers, or, if made for some 

 special purposes, as alcoholometers, urinometers, alkalimeters, lactometers, etc. 



Hydrometers are instruments generally made of glass tubes, 

 having a weight at the lower end to maintain them in an upright 

 position in the fluid to be tested as to specific gravity, and a stem 

 above, bearing a scale. The principle upon which their construction 

 depends is the fact that a solid substance when placed in a liquid 

 heavier than itself displaces a volume of this liquid equal to the 

 whole weight of the displacing substance. The hydrometer will 

 consequently sink lower in liquids of lower specific gravity than in 

 heavier ones, as the instrument has to displace a larger bulk of liquid 

 in the lighter than in the heavier liquid in order to displace its own 

 weight. 



"Weight of gases. We have so far considered the gravity of solids 

 and liquids only, and the next question will be : Do gases also possess 

 weight are they also attracted by the earth ? The fact that a gas, 

 when generated or liberated, expands in every direction, might indi- 

 cate that the molecules of a gas have no weight, are not attracted by 

 the earth. A few simple experiments will, however, show that gases, 

 Jike all other substances, have weight. Thus a flask from which the 

 atmospheric air has been removed will weigh less than the same flask 

 when filled with atmospheric air or any other gas. 



