xv THE ATMOSPHERE 231 



CHIEF POINTS OF CHAPTER XV 



Iron gains in weight during rusting, taking some material from the 

 air, and this material is the part of the air concerned in burning. 

 These facts were found out and established by the series of experi- 

 ments which the reader has, it is presumed, either performed or seen 

 performed. 



Nitrogen is the inactive part of the air. It makes up 80 per cent, 

 (roughly) of the atmosphere. It is a very inert element. It does 

 not burn, nor will it allow other things to burn in it. It can be 

 made to unite with a few metals, such as magnesium and lithium, 

 forming nitrides. 



Argon is another very inert element, which was discovered in 1894. 

 It makes up about 1 per cent, of the air. 



Oxygen is the active part of the air. When red oxide of mercury 

 is heated it decomposes into mercury and oxygen. This gas can 

 also be obtained by heating potassium-chlorate. The oxygen is 

 given off from this substance more readily if it is mixed with man- 

 ganese dioxide before being heated. The substance left after driving 

 the oxygen out of potassium chlorate by heating it is called potas- 

 sium chloride and is very like common salt. 



Oxides. When some elements are heated in oxygen they unite 

 with it, forming oxides. Thus 



Iron and oxygen form Oxide of iron 

 Phosphorus and oxygen form Phosphorus pent-oxide 

 Carbon and oxygen form Carbon di-oxide 

 Sodium and oxygen form Sodium oxide. 



Some oxides unite with water to form acids which turn blue litmus 

 red ; the oxide of phosphorus is an example. 



Other oxides unite with water to form an alkaline solution which 

 has a soapy feel ; the oxide of sodium is an example. 



Quantitative Character of Chemical Action. It has been found 

 that when the experiment of heating potassium chlorate is carefully 

 performed a given quantity of this substance always gives the same 

 quantity of oxygen. 



Various Substances present in the Air. Carbon dioxide is generally 

 present to the extent of 0'04 per cent. , or 4 parts of carbon dioxide to 

 10,000 of air. This carbon dioxide is a very important part of the 

 food of plants. Its presence is partly due to the breathing of animals. 



Moisture is always present. Its amount varies with the tem- 

 perature. It is often deposited as dew. 



Ammonium compounds, oxides of sulphur, dust particles, and 

 bacteria are to be found in the air in the neighbourhood of towns. 



Gain of Weight during Burning. It has been seen by performing 

 experiments with phosphorus, sulphur and magnesium, that there 

 is always a gain in weight when a substance is burnt. Thus, 

 phosphorus gains about 128 per cent., sulphur 100 per cent., and 

 magnesium 66 '5 per cent. 



Combustion and Busting. These are essentially similar. The only 

 difference is that combustion is attended with the production of 

 light, due to the high temperature to which the products are raised 

 by the heat of the chemical action. 



