208 Ozone and the Atmosphere. 



of alizarine or phenolpthalein in place of litmus. The alizarine 

 was prepared by dissolving the crystals sublimed from commer- 

 cial alizarine, in 80 p. c. alcohol. 



Papers impregnated with this solution were dried, moistened 

 with a 1 p. c. potassium iodide solution, and dried again. The 

 latter were exposed in strips, along with non-iodized alizarine 

 papers. The phenolpthalein was made by heating 3. 3 grms. 

 sublimed pthalic anhydride with 6. 6 grms phenol and 1. 5 grms. 

 sulphuric acid at 120° — 130° C, for 3 hrs., thoroughly wash- 

 ing the brown resin obtained, which amounted to about 3 

 grms.,* and dissolving in alcohol. 



The phenolpthalein potassium iodide tests were prepared in 

 a manner similar to those of alizarine. It was found, however, 

 that neither of these tests, especially the phenolpthalein, was 

 as sensitive as that made with properly prepared litmus ; and 

 their use, after some trials, was therefore abandoned. 



IV. Those depending upon the oxidation of ■ a metal or 

 metallic compound, with the development of a corresponding 

 change of color. 



The most important ozonoscopes in this class, which have 

 previously been studied, are silver, thallous hydrate, manga- 

 nous sulphate, and plumbic sulphide. Silver-leaf immersed in 

 an atmosphere containing a sufficient percentage of ozone, is 

 very slowly affected, if at all, except in the presence of moisture. 

 Under these circumstances, its surface is energetically oxidized, 

 with the formation of magnificent yellow, blue, and other 

 films, passing into black at those points at which the formation 

 of argentic oxide has reached a maximum. 



Unfortunately, silver is not sufficiently sensitive to ozone. In 

 a vessel containing in every litre of oxygen 1 mgrm. of ozone, 

 the potassium iodide, the iodo-starch tests, the thallous oxide? 

 Houzeau's, and some other tests were affected, before a notable 

 change had occurred upon a moistened silver surface.f It is 

 stated by Houzeau, that when a litre of oxygen containing 

 about 1 centigramme of ozone was passed over silver,, blacken- 



* See Bayer, Ber. der Beutseh. Chem. Gesell., 1871, p. 658. 

 | See also Fremy, Compt. Bend., 61, 939. 



