COMPOSITION OF AIR 



17 



Should the air in the tube diminish in volume in the course of a 

 few hours, it must be concluded that the rust is the result of the 

 absorption of air by iron. 



Continue the experiment long enough to determine whether 

 the air is entirely absorbed, and, if not, what are the propor- 

 tions of the active and inactive constituents. Should only a 

 portion of the air be absorbed, ascertain whether the unab- 

 sorbed part differs from the original air in any respect other 

 than its inability to cause iron to rust. For instance, try 

 whether it still supports the combustion of a taper. To do 

 this, cover the orifice of the tube with a glass plate while under 

 water, then remove the tube, turn up, and, having removed the 

 plate, insert a lighted taper. 



It is very important that (i) the object of each experiment, (2) the 

 drawing and description of the apparatus, (3) every detail of the observa- 

 tions made, and (4) the conclusions arrived at, should be recorded in the 

 student's notebook while the investigation is in progress. Weighings 

 should be entered thus : — 



Capsule and iron filings taken . , = 10.543 grams. 



Capsule and iron filings after rusting . = 10.568 grams. 



Increase of weight . . . . = .025 grams. 

 The measurements of volume of air should be recorded thus : — 



Air in tube at 10 o'clock = 200 c.c. (experiment begun). 



and the volumes of active and inactive constituents per cent, of air should 

 then be calculated. 



Should the preceding experiment indicate that the air, 

 deprived of that portion which combines with the iron to 

 form rust, no longer supports the combustion of a taper, it 

 must be surmised that rusting and burning are phenomena of 



