OZONE. 227 



long and four inches square, hung against a tree, I have 

 found as good as any other more expensive arrangement. 

 After proper exposure the color must be noted. I have 

 adojDted the scale of ten sliadings, from the lightest tint to 

 the darkest brown we ever obtain. This is the same scale 

 adopted in Eyirope. Several exposures in diflerent man- 

 ners at the same time are desirable as by comparison 

 your result becomes more valuable. 



This test has been condcmed by man}', and I must ad- 

 mit that it requires attention to remedy its defects. The 

 most o-rave is the combination of iodate in the salt used 

 which may be detected by tartaric acid. This disengages 

 from the iodate hvdriodic and iodic acids, which immedi- 

 ately react on each other producing free iodine and water ; 

 while tartaric acid has no immediate cifect upon the io- 

 dide beyond the slow formation of hydriodic acid. This 

 if long exposed will combine with oxygen from the at- 

 mosphere producing water and free iodine. When, how- 

 ever small a trace of iodate is present discoloration 

 immediately takes place proportionate to the amount. 

 Again dampness hastens the discoloration and so the rel- 

 ative humidity should be taken into account. Iodate of 

 potassium may form on your test strips and prevent dis- 

 coloration, being exactly equal to what would have been 

 set free by the ozone, provided no iodate had been formed. 

 This may be counteracted and the iodine set free by a 

 spray thrown upon the test from a solution of tartaric 

 acid by a common atomizer and you will then have the 

 proper shade. 



I have entered this much into detail to show how ob- 

 servers by want of proper care may have fallen into er- 

 rors, and thus have arisen the contradictory opinions ex- 

 pressed by various writers. What ozone is has but 

 recently been established ; but thanks, to Andrews an(^. 



