222 Domestic Science 



150. By carrying out experiments similar to the 

 last experiment very carefully, the percentage weights 

 of oxygen and nitrogen present in air have been found 

 to be 



Nitrogen . . . . 77 per cent. 



Oxygen . . . . 23 per cent. 



As a result of many careful experiments on the 

 percentage composition of air by volume, the following 

 numbers have been obtained : 



Nitrogen . . . . 7 9' 04 per cent. 

 Oxygen . . . . 20'96 per cent. 



(Argon is here included in each case with the nitro- 

 gen, and certain other constituents of ordinary air, 

 which are dealt with in Chapter xiv, are removed before 

 the analyses are made.) 



QUESTIONS 



1. What do you understand by the term " oxide " ? Name 

 three oxides, and give directions for the preparation of a specimen 

 of each. 



2. Describe fully how you would show that air consists of at 

 least two portions, one of which takes part in the burning of a match, 

 while the other does not. 



3. A quantity of black powder is placed in a porcelain crucible, 

 and both crucible and contents are carefully weighed. They are 

 then heated to a bright red heat in a furnace for ten minutes. On 

 cooling, the powder is found to be of a dark brown colour, and on 

 weighing crucible and powder, their combined weight is now greater 

 than before. What is the probable explanation of these changes ? 



4. A piece of phosphorus is burnt in a closed vessel of air standing 

 over water. The volume of the vessel is 1245 c.c. What volume 

 of gas is left after the phosphorus has ceased burning and the 

 apparatus has cooled ? 



