208 



SCIENCE PROGRESS 



lecture experiment to show this because water containing 

 dissolved oxygen has the same colour and appearance as 

 absolutely air-free water. But the principle may be made clear 

 by a striking experiment with copper. 



Some cuprous chloride is dissolved in strong hydrochloric 

 acid and allowed to turn black by absorption of oxygen from 



Fig. 9. 



Fig. 10. 



the air. The solution is transferred to a narrow rectangular 

 glass tank and a piece of copper suspended in it by a glass rod. 

 The top of the tank may be loosely covered with glass plates. 

 In the course of a few hours or days, depending upon the 

 strength of the acid, the liquid becomes clear around the copper, 

 indicating that the oxygen has been removed. This clear 

 portion corresponds exactly to the corrosion zone in the case 

 of iron, and the effect is decidedly pretty. Some idea of it may 

 be obtained from the photograph (fig. 11), which shows a con- 

 dition of equilibrium closely corresponding to that indicated in 

 fig. 10. 



The ideal condition for testing the rate of corrosion of a 



Fig. 12. 



piece of metal is shown in fig. 12, and the probability is that 

 if / is the length of the iron plate or the diameter of the iron 

 sphere employed, the distance of the metal from the side of the 

 containing vessel ought not to be less than about 2 /. 



