142 ELEMENTS OF SCIENCE 



odour we experience when sulphur is burnt ; various 

 bodies formed by it in conjunction with certain other 

 matters, are termed sulphides. 



Phosphorus can, like sulphur, exist in two distinct 

 solid states. One of these also is crystalline ; when in 

 its not crystalline state, it is said to be in an amorphous 

 condition. It has a great affinity for oxygen, and 

 readily bursts into flame, while ordinarily it is in a state 

 of slow combustion which makes it luminous in the dark. 

 In union with oxygen it forms phosphorus and phos- 

 phoric acid, the latter containing, of course, the greater 

 proportion of oxygen. Phosphoric acid when united 

 with other substances forms what are called phosphates, 

 as, e.g., phosphate of lime. 



Iodine exists in sea-water. When pure it is a soft, 

 opaque, crystalline solid, of a bluish black colour and 

 with a metallic lustre. When moderately heated it 

 becomes a violet coloured vapour, which solidifies again 

 in crystals. It has a strong, disagreeable odour and 

 taste, and gives an intense blue colour to a solution of 

 starch. It unites with metals, forming what are called 

 Iodides. 



Of the metals, gold and silver do not rust (oxidise) 

 by exposure to the air, and they, with platinum, mer- 

 cury and copper, are often found pure, or in their 

 " native" state, as it is called. 



It is just the reverse with the metals potassium,* 

 calcium, aluminium, and also sodium, the oxide of 

 which is soda. Magnesium oxidises as magnesia. /Silicon, 

 or silicium, unites with oxygen, as before said,f to form 

 silica i.e., silicic acid, and the products of this acid with 

 other bodies are termed silicates. Slate, much of what 



* See ante, p. 132. f See ante, p. 131. 



