AIR AND OXYGEN 



37 



FlG 21 



we should know how the necessary measurements are made. The 



most exact measurement of the proportions in which the elements 



combine to form compounds involves manipulations too elaborate 



to be gone into here. One or two brief statements, diagrammatic 



rather than accurate, will 



show the principles, how- " <: $&$f" ' 



ever. 



If we take a weighed 

 quantity of iron in a test- 

 tube and heat it with more 

 than enough sulphur (an 

 excess of sulphur), we get 

 free sulphur along with 

 the ferrous sulphide (p. 



15), and no free iron survives. We may remove the free sulphur 

 by washing the solid with carbon disulphide. The difference be- 

 tween the weights of the ferrous sulphide and the iron gives the 

 amount of sulphur combined with the known quantity of the 

 latter. 



As an example of the study of the combination of a metal with 

 oxygen, we may weigh a small amount of copper in the form of 

 powder in a porcelain boat and pass oxygen over the heated metal 

 until it is completely converted into black cupric oxide (Fig. 

 21). The original weight of the copper, and the increase in 

 weight, representing oxygen, give us then the data for determin- 

 ing the composition of this oxide. The data furnished by one 

 rough lecture-experiment, for example, were as follows: 



Weight of boat + copper 4.278 g. 



Weight of boat empty 3 . 428 g 



Difference = weight of copper 0. 860 g. 



Weight after addition of oxygen 4 . 



Weight without oxygen 4 . 278 g. 



Difference = weight of oxygen 0. 210 g. 



