Schoene has given the results of an extended series of experiments on the
use of thallium paper for estimating approximately the oxidizing material
in the atmosphere, whether it be hydrogen peroxide alone, or mixed with
ozone, or perhaps also with other constituents hitherto unknown. The
objection to Schönbein's ozonometer (potassium iodide on starch paper) and
to Houzeau's ozonometer (potassium iodide on red litmus paper) lies in
the fact that their materials are hygroscopic, and their indications vary
widely with the moisture of the air. Since dry ozone does not act on these
papers, they must be moistened; and then the amount of moisture varies the
result quite as much as the amount of ozone. Indeed, attention has been
called to the larger amount of ozone near salt works and waterfalls, and
the erroneous opinion advanced that ozone is formed when water is finely
divided. And Böttger has stated that ozone is formed when ether is
atomized; the fact being that the reaction he observed was due to the
H_2O_2 always present in ether. Direct experiments with the Schönbein
ozonometer and the psychrometer gave parallel curves; whence the author
regards the former as only a crude hygrometer. These objections do not lie
against the thallium paper, the oxidation to brown oxide by either ozone or
hydrogen peroxide not requiring the presence of moisture, and the color,
therefore, being independent of the hygrometric state of the air. Moreover,
when well cared for, the papers undergo no farther change of color and may
be preserved indefinitely. The author prepares the thallium paper a few
days before use, by dipping strips of Swedish filtering paper in a solution
of thallous hydrate, and drying. The solution is prepared by pouring a
solution of thallous sulphate into a boiling solution of barium hydrate,
equivalent quantities being taken, the resulting solution of thallous
hydrate being concentrated in vacuo until 100 c.c. contains 10 grammes
Tl(OH). For use the strips are hung in the free air in a close vessel,
preferably over caustic lime, for twelve hours. Other papers are used, made
with a two per cent. solution. These are exposed for thirty-six hours. The
coloration is determined by comparison with a scale having eleven degrees
of intensity upon it. Compared with Schönbein's ozonometer, the results are
in general directly opposite. The thallium papers show that the greatest
effect is in the daytime, the iodide papers that it is at night. Yearly
curves show that the former generally indicate a rise when the latter give
a fall. The iodide curve follows closely that of relative humidity, clouds,
and rain; the thallium curve stands in no relation to it. A table of
results for the year 1879 is given in monthly means, of the two thallium
papers, the ozonometer, the relative humidity, cloudiness, rain, and
velocity of wind.--_G. F. B., in Ber. Berl. Chem. Ces._
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