206 Ozone and the Atmosphere. 



filtrates afforded no trace of saline matters, and then dried at 

 the temperature of the air. 



Paper : — A great variety of papers was examined, and all 

 rejected for one reason or another, except the best Swedish 

 filter-paper. This appeared to have no action on the tests, ex- 

 cept possibly upon the thallium and the Moffat. The former, 

 after keeping for some time, was minutely spotted with thallic 

 oxide, and the latter turned brown on the edges. But it is 

 probable that, in the latter case at least, the alteration of the 

 ozonoscope was due to other causes than the paper. The 

 paper might have been treated with dilute acid, and then re- 

 peatedly washed with, distilled water, in order to remore all the 

 impurities soluble in these menstrua ; but this was not done 

 for fear that the paper might be made too fragile to stand 

 much subsequent handling. But tine cotton cloth, treated in 

 this manner, might answer for certain of the tests, better than 

 paper. 



Classification of Ozonoseopes : — The most important ozono- 

 scopes hitherto in use, may be conveniently grouped into five 

 classes : 



I. Those depending on the decomposition of a metallic 

 iodide, the liberated iodine indicating the reaction. Under 

 this head, the substance almost exclusively used is iodide of 

 potassium. An examination of the tables given below, will 

 show that it is the most sensitive of ail the ozonoscopes exam- 

 ined. Unfortunately, the action of ozone does not stop with 

 oxidation of the potassium, but extends to the iodine, so that 

 a paper once brown from free iodine, may become quite color- 

 less again, from the formation of potassium iodate. It is pro- 

 posed to examine other haloid compounds, especially the 

 iodides of the heavy metals. 



II. Class No I, with starch added as an indicator. Very 

 many tests, included in this class, have been proposed. They 

 differ chiefly in the relative proportions of potassium iodide 

 and starch. With some, the comparison is made when dry, in 



