duction of free iodine and potassium hydrate from potassium iodide; (2) The oxidation of thallous to thallic oxide; (3) The oxidation of manganous to manganic oxide.
The first of these is the test most commonly employed. A paste is prepared by adding 10 parts of the best quality of starch to 200 parts of pure water, heating this until the starch gelatinizes, and then dissolving in it 1 part of pure iodide of potassium. This paste is then spread evenly on sheets of paper free from sizing, which are then rapidly dried without exposure to sunlight.
A slip of this paper 1/2 inch wide and 4 inches long is moistened in pure water and suspended where it will be screened from the sun, but exposed to diffuse daylight and to the air. The inside of the standard instrument shelter of the Weather Bureau is probably the best exposure obtainable. After being exposed for from eight to twelve hours the paper is taken down and dipped in water, and its color compared with a standard ozone scale of colors, arranged on a scale of 1 to 10.
The objection to this method is, as shown by Schönbein, that hydrogen peroxide (H2O2) as well as ozone reacts upon this paper, which is also hygroscopic, its indications varying with the relative humidity of the atmosphere.
Liquid Air. --Within recent years every known gas has been liquefied, including helium. Most matter can be readily changed from the solid to the liquid state, and from the liquid to the gaseous by the application of heat. The problem of the liquefaction of gases is mainly one of producing extreme cold. This is now best accomplished by causing a gas that has been cooled by expansion to circulate about the pipes containing the gas that is to be liquefied. Liquid air has about the same color and specific gravity as water. At ordinary pressure it boils at -312° F.
HANN J., "Handbook of Climatology," translated by R. DeC. Ward, New York, etc., 1903, pp. 74-83.
RENK FRIEDRICH, "Die Luft," Leipzig, 1886. ( Pettenkofer & Ziemssen, "Handbuch der Hygiene und der Gewerbekrankheiten," 1. Teil; 2. Abteilung; 2. Heft.)
RAMSAY SIR WILLIAM, "The Gases of the Atmosphere; the History of their Discovery," 3d ed., London, 1905.
"Smithsonian Meteorological Tables," Edition of 1907, Washington, D. C.