342 



Experiments on the Production of Ozone. [May 6, 



sides of the box, and a black velvet cloth was then pat over the box 

 so that all stray light was excluded, and any spark traversing the box 

 could easily be detected. The air was sucked through the box at the 

 rate of about a litre in ten minutes. Before trying the experiment 

 air was sucked through for about half an hour when the electrodes 

 were at the same potential ; but not the slightest colouration of the 

 potassium iodide solution in the test-tube or on the pieces of paper in 

 the box could be detected. We then adjusted the distance between 

 the terminals of the Holtz machine so that the sparks just did not 

 pass across the vessel, in this case the terminals of the Holtz were 

 about 4 inches apart, so that the field was as intense as it could be 

 without producing a discharge. Air was then sucked through for 

 more than an hour, but not the slightest colouration could be detected 

 in either the test-tube or the pieces of paper, though the passage of a 

 single flash was sufficient to produce a most distinct colouration. 

 This experiment was repeated over and over again, but always with 

 the same result ; we never found any ozone unless we had previously 

 seen a flash across the vessel, hence we conclude that ozone is only 

 produced when sparks pass through the oxygen. 



A special experiment was made in order to estimate the delicacy of 

 the test for ozone : to the same quantity of the solution of iodide of 

 potassium as that through which the air bubbled on its way out from 

 the vessel, chlorine-water was added until we could detect a discolour- 

 ation. The amount of chlorine in the quantity of chlorine-water 

 added was then determined by finding the quantity of iodine set free 

 by 10 c.c. of it. This was done by means of some very carefully 

 prepared solution of sodium hyposulphite, kindly made up and 

 standardised for us by Mr. M. M. Pattison Muir. From the 

 minimum quantity of chlorine required to produce a discolouration 

 of the solution, we found that the smallest quantity of ozone we 

 could detect with certainty was O0384 mgram. But 6 litres of air, that 

 is 1*5 litres of oxygen, had passed slowly through the apparatus and, 

 since no discolouration was produced, the amount of ozone formed 

 must have been less than 00384 mgram., or less than 0*00016 of the 

 whole quantity of oxygen which had passed through the apparatus. 

 In the second experiment we took an ozonizer made of two concentric 

 tubes, and sealed up in it air free from ozone and a large quantity 

 of phosphorous pentoxide, this was left for three months, so that at 

 the end of the time the air was presumably dry; on causing the 

 electric discharge to pass through it, however, ozone was produced in 

 large quantities, so that ozone is produced when an electric spark 

 passes through very carefully dried oxygen. 



