48 Dr. E. H. Cook on Experiments 



very soon became rapidly charged, and this seemed to pre- 

 vent the production of the brush. 



Experiments with Potassium Iodide. 



The chemical action produced by the brushes in air may be 

 the formation of ozone, the production of oxides of nitrogen, 

 and perhaps other less known combinations. Potassic iodide 

 would be decomposed by ozone and the oxides of nitrogen, 

 and it is therefore a suitable substance to experiment with. 

 After many trials the simplest apparatus was found to be the 

 most useful. The points attached to the positive and negative 

 sides of the machine were placed at such a distance above the 

 surface of some standard potassic iodide solution that no 

 actual spark from the point could pass to the solution (with 

 the machine and potential used, this was anything beyond 

 2 centim.). This precaution was taken in order that no breaking 

 up of the iodide should take place in consequence of the 

 spark passing through it. The solution was contained in glass 

 or porcelain dishes, and as solution of potassic iodide (unless 

 perfectly pure) becomes slightly coloured when exposed for 

 some time to air and light, a similar quantity of the standard 

 solution w 7 as placed in an extra dish of equal size at the same 

 time. At the conclusion of the experiment the amount of 

 iodine set free from this blank was estimated and subtracted 

 from that produced under the brushes. 



The results obtained have differed among themselves as far 

 as actual amounts of iodine set free at one brush compared 

 to the amount set free at the other, but they have all agreed 

 in this important particular, that the amount of iodine produced 

 by the negative brush from the machine is always very much 

 greater than the amount produced by the positive in the 

 same time. 



With points prepared as similarly as possible, and every 

 precaution taken to avoid leakage, it was found that from, five 

 to eight times as much iodine was set free by the negative 

 brush as by the positive. The following are details of a 

 typical experiment : — 



Speed of rotation of plates of machine, 300 per minute. 



Potential-difference about 40,000 volts. 



Distance of points from surface of solution, 4 centims. 



Amount of iodine liberated by the negative brush over that 

 in blank in half-hour, 0*000762 gramme. 



Amount of iodine liberated by the positive brush over that 

 in blank in half-hour, 0*000 127 gramme. 



The greatest amount of iodine liberated in one hour in this 

 way amounted to 0'001778 gramme ; assuming this to be 



