316 



SCIENTIFIC RECREATIONS. 



in water, and makes a peculiar crackling when thrown in the fire. But 

 though its principal physical properties may be familiar enough, many people 

 are entirely ignorant of its chemical nature and elementary composition. 

 Kitchen salt contains a metal, combined with a gas possessing a very suffo- 

 cating odour ; the metal is sodium^ the gas is chlorine. The scientific name for 

 the substance is chloride of sodium (salt).* The metal contained in common 

 salt in no way resembles ordinary metals ; it is white like silver, but tarnishes 



Fig. 304. Pharaoh's serpent. 



immediately in contact with air, and unites with oxygen, thus transforming 

 itself into oxide of sodium. To preserve this singular metal it is necessary 

 to protect it from the action of the atmosphere, and to keep it in a bottle 

 containing oil of naptha. Sodium is soft, and it is possible with a pair of 



* It is the same with a number of other common products, such as clay, sandstone, etc., 

 the composition of which chemistry has revealed. Argil, or clay, slate, and schist all contain 

 a metal aluminium, which has become most valuable for industrial purposes. Stones for 

 building are composed of a metal combined with carbon and oxygen calcium; sandstone 

 is composed of silicium, a metallic body united with oxygen ; and sulphate of magnesia, 

 -which enters into the composition of a purgative drink, also contains a metal magnesium. 



