LESSONS IN PHYSICS. 37 



of the two being 476 grains, which is the weight of a volume of 

 water equal to the volume of both bodies. Then weighing the 

 lead by itself in water, it is found to lose 152 grains, and this 

 subtracted from 476, the loss of both, gives 324 grains as the 

 weight of the water displaced by the lighter body. Then dividing 

 the weight of the lighter body in air (200 gr.) by the weight of 

 an equal bulk of water (324 gr.) gives the specific gravity of the 

 lighter body. From this data the volume of the lighter body 

 may be found as before. 



To do this kind of work accurately, delicate scales and pure 

 water of definite temperature, are necessary ; but with a home- 

 made balance, using bird-shot for weights, results of great accur- 

 acy may be obtained . Water is the standard in finding the specific 

 gravity of liquids as well as solids. We may find the specific 

 gravity of a liquid by weighing a given volume of the liquid and 

 dividing that weight by the weight of an equal volume of water. 

 To facilitate such operations specific gravity bottles are made, 

 holding 1,000 grains of pure water, so that one has only to fill 

 the bottle with the liquid he wishes to test, and weigh it and 

 divide the result by 1,000, to get the specific gravity. In this 

 way we compare the weight of a volume of the liquid to be tested 

 and the weight of an equal volume of the standard that is, 

 we compare weights of equal volumes but the same result 

 may be reached by comparing the volumes of equal weights, 

 which is the principle of the hydrometer, lactometer, alcohol- 

 meter, etc. These instruments are made so as to sink to a 

 given depth in pure water, milk, or alcohol. If the liquid tested 

 is lighter than the standard the instrument sinks deeper if 

 heavier it does not sink as far and the difference is read from 

 the scale. This principle may be illustrated by using a piece of 

 wood about an inch square by 6 or 8 inches long, into one end 

 of which a nail has been driven so that it will stand erect in a 

 liquid. Place the stick in a jar of water and note how far it sinks 

 then put it in a jar of milk, kerosene oil, or any other liquid, 

 and note how far it sinks, and the principle of the hydrometer is 

 easily understood. 



