Ozone and the Atmosphere. 207 



others after moistening. Or the paper may contain potassium 

 iodide only, the iodide of starch being formed subsequently, 

 by moistening the papers, after exposure, with starch solution. 

 We have added to this list, one new test, containing iodide of 

 cadmium (as representative of the iodides of the heavy metals) 

 and starch. It develops the characteristic blue color of iodide 

 of starch very strongly and persistently on moistening, but is 

 less sensitive than some of the other tests. 



III. Those depending upon the alkaline reaction resulting 

 from conversion of the electro-positive element into the form 

 of oxide, — litmus or some similar coloring matter being added 

 as indicator. It will be seen from experiments detailed below, 

 that there are other bodies sometimes present in the atmo- 

 sphere, which decompose potassium iodide, as well as ozone. 

 But if this decomposition is attended with formation of caustic 

 potash, only when ozone is the decomposing agent, the litmus- 

 paper will be. turned blue only in case of the presence of ozone. 

 This was the ground taken by Houzeau, who proposed, and 

 has strenuously advocated, tests of this character. He admits, 

 however, that hydrogen proxide will likewise develop an 

 alkaline reaction. The tables, above referred to, exhibit a 

 similar result. 



The best method of preparing these tests is to saturate papers 

 with litmus solution containing about 1 centigramme of litmus 

 in 1 c. c. The litmus must be brought with great care to that 

 shade of wine-red at which it is most sensitive. After drying 

 some of these papers are impregnated with a 1 p. c. solution of 

 potassium iodide and again dried. Papers of both kinds, the 

 plain and iodized litmus, are cut into slips and exposed in pairs. 

 It is expedient to subject these tests to the action of artificially 

 prepared ozone, to see that they are really sensitive. Great 

 differences were found in this manner, between tests not appa- 

 rently differing much in color. Even the best did not appear 

 so sensitive as some of those in the first and second classes. 

 (See tables.) 



Two new tests were added to this class : — the alizarine and 

 phenolpthalein potassium iodide tests, distinguished by the use 



