ELEMENTS, COMPOUNDS, AND MIXTURES 255 



bromide and gelatine. The silver bromide is quite sen- 

 sitive to light, and when the plate is exposed and properly 

 developed a silver image of the object photographed is 

 left upon it. The " printing paper " -is essentially like 

 the plates. Some papers are washed with a silver chlo- 

 ride and white of egg emulsion, and others like "velox" 

 are rendered sensitive to light by being washed with a 

 silver bromide emulsion. 



Gold. Gold is found in a pure state in veins of 

 quartz in almost every part of the world. Occasionally 

 large nuggets of gold are found. 



Since gold is little affected by the chemical action of 

 the air, water, and other substances, it has come into 

 common use for coins and jewelry. It is alloyed with 

 silver and copper to give it hardness. Twenty-four carat 

 gold is pure gold ; eighteen carat gold is three fourths 

 pure ; and fourteen carat gold is ^J pure. The gold 

 coins of the United States are ninety per cent gold and 

 ten per cent copper. Gold is the most ductile and malle- 

 able of metals. It melts at 1075 C. It is not dissolved 

 by any single acid, but a mixture of hydrochloric and 

 nitric acids will dissolve it. 



Chlorine. -- The gases, oxygen, hydrogen, and nitro- 

 gen, which we have studied previously are colorless, 

 odorless, and tasteless ; but chlorine is a green gas having 

 a disagreeable suffocating odor. When breathed, it irri- 

 tates the lining of the nose and throat. Chlorine occurs 

 in a great many compounds, but it is found most abun- 

 dantly in common salt. It is named from chloros, a Greek 

 word meaning green. 



Chlorine is a powerful bleaching agent and has a wide 

 commercial use for this purpose. Large quantities of it 

 are used in making bleaching powder, chloride of lime. It 



