xv THE ATMOSPHERE 219 



&c. Hence it is clear that this gives us the proportion of the 

 active part of the air, and we may sum up the result of the 

 experiment as follows. 



Air contains 80 per cent, inactive part, which does not allow 

 things to burn in it, so that the remaining 20 per cent, must be 

 the part concerned in burning, and which we have called the active 

 part. To the inactive part we shall henceforth give the name 

 Nitrogen. 



This gas nitrogen is an example of a very inert element, since 

 it can only with difficulty be made to combine with any other 

 element. It does not burn, nor will it allow other substances to 

 burn hi it ; or, as usually expressed, it does not support com- 

 bustion. It can be made to unite with a few metals, as magnesium 

 or lithium. If either of these be heated with nitrogen the gas is 

 absorbed and a compound of magnesium and nitrogen called 

 magnesium nitride, or lithium and nitrogen called lithium 

 nitride, is obtained. 



If the nitrogen obtained in Experiment 210 be thus heated with 

 one of these metals it is found that about 1 per cent, of it remains 

 unabsorbed. This residue is another substance present in the 

 atmosphere to the extent indicated, and called Argon. Argon 

 is also very inert, more so even than nitrogen. Owing to this, 

 even until the year 1894, its presence in the air had been 

 completely overlooked, although, nearly a century before, the 

 eminent chemist Cavendish had unknowingly obtained some, 

 regarding it as an impurity which he had overlooked an 

 example of the importance of giving attention to the minutest 

 details in scientific investigations. 



The Active Constituent of Air. We must now endeavour 

 to obtain and examine the active part of the air which dis- 

 appeared during the rusting of the iron. Knowing that it is 

 now present in the rust, the most evident plan would be to 

 endeavour to obtain it from this source. The ease with which 

 iron rusts, that is, the readiness with which it takes up the 

 active part of the air, should indicate that it would probably be 

 very difficult to obtain it from this source, and that some other 

 rust which is more difficult to prepare would probably be better 

 for our purpose. The most convenient is the rust of mercury, 

 which is a red powder not easily formed. It can be obtained 

 if mercury is heated for a considerable time in air, when it 

 forms as a red scum upon the surface of the metal. The changes 



