218 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



Now we must ask Did the gain come from the water we 

 added, or from the atmosphere ? To answer this question it will 

 be necessary to allow the iron to rust in a closed space, so that if 

 anything is taken from the air we can notice it. 



EXPT. 210. Place some iron filings in a muslin bag and tie 

 the bag to a piece of glass rod. Moisten well (better dip in a 

 solution of sal ammoniac) and place it in a bottle of air in- 

 verted over water (Fig. 110). Examine after a few days. It 



will be seen that the water has 

 risen in the glass, showing that 

 some part of the atmosphere has 

 been abstracted by the iron in 

 rusting. 



EXPT. 211. Tightly place your 

 hand on a card under the mouth 

 of the jar so as to allow no water 

 to escape, set upright and place 

 a burning taper into the jar. 

 Note what happens, but do not 

 throw away the water. 



FIG. 110. Action of Iron Filings 

 on Air. 



The taper being extinguished 

 proves to us that the gas left in 

 the jar differs from air in the respect 

 that it does not allow substances to burn in it. Hence the 

 material taken away from the air by the iron, and which with 

 the iron formed rust, is that part of the air which is concerned 

 in burning, and we may state : 



Iron in rusting gains weight, taking some material from 

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

 burning. 



EXPT. 212. Next measure in a graduated vessel the quantity 

 of water in the bottle. This is equal to the quantity of gas 

 which has been used by the iron. Also measure the quantity 

 of water the bottle holds. This is the quantity of air it origin- 

 ally held. 



Chemical Composition of Air. It will be found by repeating 

 the experiment with different bottles at different times, and with 

 different quantities of air, that the ratio of these two volumes is 

 always roughly 1 : 5, and is independent of the size of the bottle, 



