Mr Townsend, The formation of clouds with ozone. 55 



A second tube B also led into F, and by means of it oxygen 

 which had passed over a solution in G could be brought into 

 contact with the ozone. The effect on the ozone of dissolving 

 certain substances in the solution in G was immediately apparent 

 by the formation of a dense cloud in F. Thus when G contained 

 a fresh solution of potassium iodide no cloud was formed in the 

 flask when oxygen which had passed over the solution diffused 

 through the ozone. A small quantity of iodine was then dissolved 

 in the potassium iodide, and on repeating the experiment it was 

 found that a dense cloud was formed, when oxygen, which had 

 passed over the solution of potassium iodide with iodine dissolved 

 in it, diffused through the ozone in the flask. We have therefore 

 strong evidence that the cloud is due to the action of ozone on the 

 iodine vapour which the stream of oxygen carried with it from G, 

 as neither the ozone nor the iodine vapour formed a cloud by 

 itself. The tubing leading the oxygen into G did not dip under 

 the solution, so that it is impossible to attribute the effect to 

 splashing. A similar set of experiments was performed with a 

 solution of sulphur dioxide in the vessel G, and it was found that 

 a cloud was formed when oxygen containing some sulphur dioxide 

 gas diffused through the ozone. 



4. Returning to the original method of forming these clouds, 

 by allowing the tube A to dip under a solution which acts on the 

 ozone as it is passed through, additional evidence can be obtained 

 in support of the above explanation as to the cause of the forma- 

 tion of the cloud. If the formation of the cloud requires the 

 presence of a gas formed in the solution, the cloud should be 

 much denser when an increased quantity of the gas is expelled 

 from the solution by heating it gently. When the ozone is 

 bubbled through a fresh solution of potassium iodide, the cloud is 

 not formed immediately, showing that at first the quantity of 

 iodine present in the gas is too small to give a visible effect. 

 When however the bubbling has been going on for a while and 

 the solution has acquired a red colour, a cloud is formed the 

 density of which can be greatly increased by raising the tempera- 

 ture a few degrees. The same results were obtained with a 

 solution of metabisulphite ; the cloud did not appear at first, but 

 when it began to be formed its density was greatly increased by 

 heating the solution. It was found impossible to form a cloud by 

 passing ozone through sodium sulphite and warming the solution. 



In the case of the potassium iodide solution the density of the 

 cloud is also increased by acidulating the solution with sulphuric 

 acid. This has the effect of neutralising the caustic potash which 

 is formed, when the iodine is set free, which tends to keep the 

 iodine in solution. 



