258 GENERAL SCIENCE 



Specific Gravity. A knowledge of the density or specific gravity 

 of a liquid or a solid is often important to determine the purity or 

 condition of materials. In the preparation of syrups, jellies and 

 other food products, the determination of their specific gravity is 

 a very convenient way of knowing whether the process of evaporation 

 or " boiling down " has been sufficient. 



The condition of milk as to its fat content may be determined by 

 measuring the specific gravity of milk, which is from 1.028 to 1.032. 

 If the specific gravity is above this, the probability is that cream has 

 been removed. 



The purity of gasoline may be determined by its specific gravity. 

 Gasoline has a specific gravity of 0.70 to 0.74. If the specific 

 gravity of the gasoline is different from this we know that the 

 gasoline is impure, and not of standard grade. 



Electric storage batteries used in automobiles, etc., are tested 

 as to their specific gravity to determine whether they are fully 

 charged. A battery reading 1.300 is overcharged, while a battery 

 reading 1.280 is fully charged. If the reading is below this specific 

 gravity, the battery is undercharged. 



Meaning of Specific Gravity. Specific gravity is the relation 

 between the weight of a substance and that of an equal volume of water. 

 For example, zinc has a specific gravity of 7, which means that a 

 cubic foot, cubic inch, or cubic centimeter of zinc is 7 times heavier 

 than a cubic foot, cubic inch or cubic centimeter, respectively, of 

 water. The weight of water is taken at 4 C. or about 39 F. 

 (Refer to page 114) Therefore, a good definition of specific gravity 

 is: the ratio of the weight of any volume of a substance to the 

 weight of an equal volu/ne of water. 



The human body has a specific gravity of 1.07, which means 

 that a person is slightly heavier than water. A cubic foot of wate; 

 weighs 62.5 pounds per cubic foot. The human body then would 

 weigh 66.8 pounds per cubic foot. 



Fleeting and Sinking Bodies. Place a wooden ball in a dish of water. 

 It will be seen to sink to a certain depth and then float. Place a metal ball 

 of the same size in water. It will sink. Weigh the metal ball out of water 

 (using a spring scale). Weigh the metal ball in water. It will be seen that 

 the metal ball has lost weight. If possible, obtain a larger piece of metalj 



