Ozone and the Atmosphere, 209 



nig took place, but when the same amount of ozone was diluted 

 with 50 litres of oxygen, the silver was unaffected.* Even the 

 latter mixture contains a far larger amount of ozone than is 

 ordinarily present in the atmosphere, if we accept the determi- 

 nations of Pless and Pierre, f who found, by titration of the 

 iodine set free on the surface of potassium iodide paper, that 

 255 litres of air contained but 0.02 mgrms. ozone. Zenger 

 found in 100 litres of air from 0.002 to 0.01 mgrm. ozone4 



For the present, at least, silver is inapplicable as an atmo- 

 spheric ozonoscope, not from want of reliability, bat from lack 

 of sensitiveness ; and the desideratum is to find some ozono- 

 scope which will be similar to silver in its reactions towards the 

 compounds of the atmosphere, and at the same time of adequate 

 sensibility. 



The next most important reagent of this class is Thallous 

 Hydrate.§ Small bars of metallic thallium were dissolved in 

 dilute sulphuric acid, the reaction being hastened by the intro- 

 duction of strips of platinum foil, and thallous sulphate crystal- 

 lized out in brilliant prismatic needles. A solution of this salt 

 was exactly neutralized with baryta, the filtrate from the ba- 

 rytic sulphate containing 10 p. c. of thallous hydrate. Strips 

 of filtering paper, immersed in this filtrate, were dried over 

 caustic potash under a bell-jar, so as to prevent them, as far as 

 possible, from absorbing carbonic anhydride from the air. 



When the ozonoscopes are compared dry, thallous hydrate 

 occupies a very high position, sometimes the 4th or 3d place, 

 but usually the 2nd position. After moistening, the papers of 

 the first three classes exhibit a more striking appearance than 

 the thallium peroxide. This result differs from that obtained 

 by ELuizinga, who thought that the thallous hydrate was more 

 sensitive than the iodo-starch tests, when the papers were 

 freshly prepared and the solution sufficiently concentrated.! 



On examining into the behavior of thallous hydrate, when 



* Comptes Kendus, Dec. 18, 1865. 



t Jahresb. der Chemie, 1857, 79. 



t Zenger, Wien. Akad-. Ber., 24, 78. 



§ Bottger, J. pr. Chem., 95, 311. 



|| Huizinga, Journal fur praktiscne Cbemie, ci, 32J. 



