48 LETTEES OF BEBZELIUS 



negative polarization or the electrical smell has 

 become perceptibly strong, a moist or air-dried strip 

 of litmus paper plunged in the air is fairly rapidly 

 bleached and tincture of litmus or indigo solution is 

 decolorized, exactly as it is by chlorine. If such air 

 is shaken with potassium ferrocyanide solution the 

 liquid becomes intensely yellow, and when treated 

 with pure ferrous sulphate gives a blue precipitate. 

 Potassium iodide solution shaken with it is turned 

 yellow, and moist starch paste mixed with this salt 

 instantly becomes blue. 



I may mention here that strips of paper soaked in 

 starch solution containing potassium iodide form the 

 most convenient and the most delicate test for ozone ; 

 they indicate its presence even when a very sensitive 

 galvanometer fails to do so, or when the smell cannot 

 be perceived. 



Just one example to prove the truth of this state- 

 ment. A small piece of phosphorus still covered 

 with moisture will not turn blue a piece of test 

 paper held above it, but if the phosphorus is rubbed 

 with blotting paper, with gentle friction, the paper 

 becomes decidedly blue in the course of a few seconds. 

 It is a remarkable fact that very small quantities of 

 ether or alcohol vapour, olefiant gas, as well as some 

 other vapours and gases, such as sulphurous acid, 

 hydrogen sulphide, etc., entirely prevent the forma- 

 tion of ozone in atmospheric air ; and this is the 

 action generally of all substances which arrest the 

 luminescence or slow combustion of phosphorus in 

 the air. I must here mention that in pure oxygen 



