326 



SCIENTIFIC RECREATIONS. 



certain conditions it can dissolve great quantities of this gas. If it is fused 

 in a small bone cupel, in contact with the air, and left to cool quickly, it 

 expands in a remarkable manner, and gives off oxygen. 



Nitric acid dissolves silver very easily, by causing the formation of 

 abundant fumes. When the solution evaporates, we perceive white crystals 

 forming, which are nitrate of silver. This fused nitrate of silver takes the 

 name of lunar caustic, and is employed in medicine. Nitrate of silver is 

 very poisonous ; it possesses the singular property of turning black under 

 the action of the sun's rays, and is used in many curious operations in 



Fig. 315. Jupiter's Tree. 



photography. It is also employed in the manufacture of dyes for the hair ; 

 it is applied to white hair with gall-nut, and under the influence of the light 

 it turns black, and gives the hair a very dark shade. Salts of silver in 

 solution with water have the property of forming a precipitate under the 

 influence of chlorides, such as sea salt. If a few grains of common salt are 

 thrown into a solution of nitrate of silver, it forms an abundant precipi- 

 tate of chloride of silver, which blackens in the light. This precipitate, 

 insoluble in nitric acid, dissolves very easily in ammonia. 



Platinum, which is the last of the precious metals that we have to 

 consider, is a greyish-white colour, and like gold is only affected by a 

 mixture of nitric acid and hydrochloric acid. It is the heaviest of all the 



