GRAVITATION. 29 



Weight is an expression used to denote the quantity of mutual 

 attraction between the earth and the body weighed. Here again, the 

 attraction of the substance for the earth is not taken into considera- 

 tion. All our weighing is a comparison with, or measurement by, 

 some standard weight, such as pound, ounce, gramme, etc. 



Specific weight or specific gravity denotes the weight of a body, 

 as compared with the weight of an equal bulk or equal volume of 

 another substance, which is taken as a standard or unit. The word 

 density is frequently used for specific weight, as density means 

 comparative mass. By the density of a body consequently is meant 

 its mass (or quantity of matter) compared with the mass of an equal 

 volume of some body arbitrarily chosen as a standard. The standard 

 or unit adopted for all solids and liquids, if not otherwise stated, is 

 water at a temperature of 15 C. = 59 F. 



Specific weight is generally expressed in numbers which denote how 

 many times the weight of an equal bulk of water is contained in the 

 weight of the substance in question. If we say that mercury has a 

 specific gravity or density of 13.6, or that alcohol has a specific gravity 

 of 0.79, we mean that equal volumes of water, mercury, and alcohol 

 represent weights in the proportion of 1, 13.6, and 0.79, or 100, 1360, 

 and 79. 



The standard or unit chosen for comparing the specific gravity of 

 gases is either atmospheric air or hydrogen. 



In order to obtain the specific gravity of any liquid, it is only 

 necessary to weigh equal volumes of water and the liquid to be ex- 

 amined, and then to divide the weight of the liquid by the weight of 

 the water. 



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 thaisa 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 



