XV 



THE ATMOSPHERE 22,5 



temperature, and this should be done, the barometric pressure 

 and air temperature being read at the time of the experiment, 

 and the volume of gas corrected for the pressure 760 mm. and 

 temperature (page 112). The weight of 1 litre of oxygen will 

 be then found as slightly over 1*4 gms. 



Summary of Results. Air contains about 80 (more ac- 

 curately 78) per cent of an inert gas, nitrogen, in which things 

 cannot burn. About 20 (more accurately 21) per cent, is a very 

 active gas, oxijgen, which supports combustion very vigorously. 

 Substances burn in it with the formation of compounds called 

 oxides. 'Some of these oxides dissolve in water and form acids, 

 while others have properties essentially different from the acid- 

 forming oxides. A little less than one per cent, of the air consists 

 of a very inert gas named Argon. 



Various substances present in Air. Besides these, which 

 are the chief substances in air, there are others which are present 

 to a small extent. The presence of some of these may be indi- 

 cated by the following experiments : 



EXPT. 224. Into a large glass flask pour a few c.c. of clear 

 lime-water. Close the jar and shake vigorously. Examine 

 the lime-water. 



EXPT. 225. Leave a few pieces of dry calcium chloride 

 exposed in a saucer to the air. Observe after a few hours. 



EXPT. 226. Lea,ve a weighed quantity of sulphuric acid ex- 

 posed to the air in a beaker for a few days. Again weigh. 



EXPT. 227. Put some ice (if not available, a little sodium 

 sulphate and hydrochloric acid) into a small beaker and watch 

 the outside of the beaker. 



In the first of these experiments we find that the lime-water 

 becomes tinted and milky. This does not happen with oxygen 

 alone (try it) or nitrogen, but we have already seen that the 

 milkiness is produced by the gas produced by burning carbon, i.e. 

 carbon dioxide. This gas is, in fact, present in the atmosphere, 

 but only to a small extent, viz., about '04 per cent., that is, 4 

 parts in 10,000. Even this minute quantity is, however, very 

 important, as plants derive the carbon which enters into their 

 composition from this source. Their green parts, under the 

 action of sunlight, decompose the oxide, and give off the 

 oxygen. 



If, instead of atmospheric air, air expired in breathing be 

 used in the above test, it will be seen that the quantity of 



Q 



