162 FIRST YEAR COURSE IN GENERAL SCIENCE 



most of the soluble salts are by this time in the springs, lakes, 

 and oceans. 



The most abundant salt, which is the chief one in the 

 ocean, is our common table salt. Its chemical name is 

 sodium chloride, because it contains sodium and chlorine. 

 It is a good example of salts, and the terms which describe it 

 would, with slight variation, describe many salts. Color, 

 degree of solubility, and the form of the crystal are important 

 physical properties of salts. (LABORATORY MANUAL, Exer- 

 cise XVIII.) 



182. Some Uses of Salts. Besides the familiar table 

 salt, there are many other salts which are used in manufac- 

 turing. 



Copper sulphate is a blue salt, commonly called blue 

 vitriol. With water, it makes a blue solution which is used 

 in electric batteries and in copper plating. 



Silver chloride and silver bromide are used in photography, 

 because light acts upon them chemically. The film or plate 

 in the camera has a coating of one of these salts. The light 

 which comes through the camera lens during the exposure 

 acts upon the silver compound. Since the light falling on 

 the film is stronger in some places than in others, the salt 

 is affected in different degrees in different places, exactly 

 corresponding with the degree of light from the objects 

 outside. To make this effect visible, the film or plate is 

 developed by treating it with solutions of various other 

 salts. Developing completes the chemical action begun by 

 light. Then another solution, the fixing bath, is used to 

 remove all the silver compounds which have not been acted 

 upon by light, and the negative is complete. It is on 

 account of this chemical effect of light on silver compounds 

 that photography is possible. 



Solutions of salts containing gold, silver, and nickel are 

 used in electroplating table utensils, jewelry, and parts of 

 machines. 



