COMBUSTION. 21 



phur,* phosphorus, t boron, J and the metals ; of these 

 hydrogen and carbon are the principal. Compound 

 combustibles are those which are formed by the union 

 of two or more simple combustibles. A candle is a 

 compound combustible, and so is coal, being composed 

 of hydrogen and carbon. 



There are also certain substances called supporters of 

 combustion, which are not in themselves combustible, 

 but whose presence is necessary for the act of combus- 

 tion : the chief of these is oxygen ; chlorines and iodine [| 

 are also supporters of combustion, but unlike oxygen, 

 they are not supporters of life. Combustion is a play 

 of affinities between the combustible^f body and the sup- 

 porter of combustion ; the combustible body has a 

 stronger affinity for the supporter of combustion than 



* Sulphur is the only simple combustible which nature offers pure 

 and in abundance. It is found in the earth, and also in depositions 

 on the surface of the earth, but most plentifully in the neighbourhood 

 of volcanoes. It is also found in combination with most of the metals. 

 It is said that 15,000 tons have been imported annually for the pur- 

 pose of making gunpowder and sulphuric acid. 



f Phosphorus abounds in the mineral, vegetable, aud animal king- 

 doms. In the mineral kingdom it is found in combination with lead 

 and iron, and more particularly with calcareous earths. In Spain there 

 are whole mountains composed of lime, combined with this substance. 

 The vegetable kingdom abounds with it, but it is principally found in 

 plants that grow in marshy places. It is also found in wheat, seeds, 

 potatoes, &c. In the animal kingdom almost every part of the body 

 contains it ; but it chiefly abounds in the bones, and it is this which 

 gives them firmness. It is also found in milk ; whence may be seen 

 the wisdom and arrangement of Providence, that this and bread should 

 form the first food of an infant, the phosphate of lime they contain 

 being indispensable in forming and strengthening the osseous parts of 

 the body. 



; Boron is a dark-coloured powder, and the base of various salts 

 called borates. The sub-borate of soda, commonly called borax, is a 

 useful ingredient in the fusion of metals. 



i For the nature of Chlorine, see the article Gaseous Bodies. 



|| Iodine is a simple substance procured from sea-weed, and was 

 first discovered in 1S12. It is of a dark grey colour, and metallic 

 lustre ; it melts and evaporates at a low heat, and its vapour is of a 

 beautiful violet-colour. 



^ A combustible body will not burn if dipped in a solution of phos- 

 phate of lime, or of muriate, sulphate, or phosphate of ammonia, with 

 boras ; the alkaline substances preventing the hydrogen combining 

 with the oxygen. 



