CHAPTER XV 



THE ATMOSPHERE 



Chemical Action. In the study of Chemistry it is advisable 

 to start with common and well-known substances, and to 

 examine these as far as possible. The knowledge so obtained 

 is then of service in the further study of other and less common 

 substances. First, therefore, from its importance and its uni- 

 versal presence, we shall investigate the atmosphere. The physical 

 properties have already been studied. We know it to be a gas 

 possessing weight and capable of exerting pressure (p. 108). We 

 have now to study its chemical properties. What is it composed 

 of ? How does it behave towards other substances ? To answer 

 these questions it will be necessary to carefully study the changes 

 which different substances undergo when exposed to the influence 

 of the atmosphere, selecting at first those changes which appear 

 simplest. Of these the rusting of metals say, iron may be 

 chosen. The first problem to be solved is, what is this rusting ? 

 Does the iron lose, or give up, anything ? Or, on the contrary, 

 does it gain anything ? To answer these questions the following 

 experiment should be done. 



EXPT. 209. Weigh carefully a watch glass with some iron 

 filings or tacks, add a few drops of water (since we know that 

 iron rusts best in the damp) and allow it to stand. At the 

 end of a few days warm gently to drive off the water, and 

 when quite dry again weigh, and note carefully the weight. 

 Has the iron gained or 'lost ? 



If the weights have been carefully taken, it will be found that 

 the iron has gained, and the experiment has furnished the neces- 

 sary answer, so that we may write : 



Iron gains in weight during rusting. 



