DENSITY AND SPECIFIC GRAVITY 



41 



equal bulks of water. The heavier the substance, the greater 

 its specific gravity is said to be. This term, however, is 

 rapidly passing out of use since the unit of mass in the metric 

 system is defined as the weight of a cubic centimeter of water 

 at its greatest density. In this system, therefore, the true 

 density of a body and its density with reference to water is 

 the same thing. The density of liquids is often measured by 

 a specific gravity bottle made of thin glass. This is first weighed 

 when empty, then when filled with water, and again when 



FIG. 9. Common hydrometer. (Duff.) FIG. 10. Specific gravity bottle ' 



filled with the liquid to be tested. In this way, the weights 

 of the two liquids are easily compared. A common way of 

 measuring the density of a liquid is by means of a hydrometer 

 which is essentially a sealed glass tube so weighted that it will 

 sink to a certain depth in pure water. In liquids more dense 

 than water, it will of course not sink so deep, and in lighter 

 liquids it will sink deeper. By means of a scale on the tube, 

 the differences and therefore the densities, are indicated. 

 Hydrometers for measuring single liquids have special names 

 which are often self-explanatory; as alcoholometer, saccharome- 



