36 PRACTICAL LESSONS IN SCIENCE. 



noting the weight, fill the pail with water, so that the stone 

 is covered, and again note the weight. The water seems to have 

 pushed upward against the stone, with a force equal to the 

 difference of weight in the two cases. Experiments show that 

 this difference between the weight of the stone in the air, and its 

 weight in water, is equal to the weight of a volume of water 

 which is equal to the volume of the stone. Now, if we divide the 

 weight of this volume of water by .577, the weight of a cubic inch 

 of water, in ounces, we get the volume of the water in cubic 

 inches, and the volume of the stone as well, however irregular the 

 stone may be. In this way the volume of any substance heavier 

 than water may be found. 



Now, if we divide the weight of the stone, in air, by the weight 

 of an equal volume of water, which is equal to the difference 

 between the weight of the stone, in air, and its weight in water, 

 we find what is called the specific gravity of the stone. 



Suppose a piece of marble weighs in the air 10 ounces ; in 

 water it weighs 6.3 ounces; the difference of weight in air and in 

 water, or the weight of an equal volume of water, is 3.7 ounces; 

 then 10 -r 3.7 = 2.7, the specific gravity of the piece of marble. 

 Now, if the specific gravity of all the more valuable substances be 

 carefully determined, we have at hand an easy method of testing 

 the purity of articles supposed to be made of gold or silver, of 

 jewels supposed to be diamonds or other precious stones, and for 

 determining the identity of many doubtful substances. The pro- 

 cess of finding the specific gravity of solids heavier than water 

 is simple, but in the case of solids lighter than water, the process 

 is more complicated, though not difficult. 



In case we wish to find the specific gravity of a body lighter 

 than water, it will be necessary to attach some heavier substance, 

 as lead, to it, in order to make it sink in the water, and then we 

 need to consider the volume of water displaced by the lead. This 

 may be illustrated by the following experiment: A body lighter 

 than water weighs in air 200 grains. When attached to a piece 

 of lead weighing 1,736 grains, they both weigh in the air 1,936 

 grains; in water they together weighed 1,460, the loss in weight 



