Page:Handbook of Meteorology.djvu/269

 water, 1 oz; starch, 25 grains; potassium iodide, 4 grains. Dissolve the potassium iodide in a small part of the water; boil the starch in the remaining part; mix and shake thoroughly. Moisten strips of unsized white paper and suspend them in the thermometer shelter or in shaded open air. Usually an exposure of from 3 hours to 10 hours is required. The ozone decomposes the potassium iodide, thereby turning the paper blue in color. This is probably the best test.

A slip of paper moistened with a solution of manganous sulphate is turned brown by ozone. The reaction oxidizes the manganous to manganic sulphate.

A paper strip smeared thinly with lead sulphide is more or less bleached by ozone, the sulphide being oxidized to a sulphate, which is white. This test is characteristic if the sulphide is smeared upon black paper.

Storm Glasses. — The so-called “storm-glass” which is sold under several fanciful names, consists of a solution just beyond the point of precipitation inclosed in a thin glass tube about 7 inches long and half an inch in diameter. The solution consists of camphor, 10 parts; potassium nitrate (saltpeter) 5 parts; ammonium chloride (sal ammoniac) 5 parts; 95 per cent alcohol 105 parts; and distilled water 45 parts. The virtue of the solution depends on the fact that neither the solution nor the precipitation is complete; the proportion of alcohol must be regulated to prevent more than a small part of the chemical salts from precipitation. The glass must be thin enough and elastic enough to yield slightly to changes in atmospheric pressure. About 1 inch of air space in the tube adds to the sensitiveness of precipitation. An increase in pressure causes the precipitation to extend nearly to the top of the solution; a decrease causes increased solution and a settling to the bottom of the tube. With a very low barometer the precipitated matter sinks to the bottom and becomes gelatinous. Increase of pressure is followed by the formation of minute crystals that grow in size. The only weather changes indicated are those foretold by a rising or by a falling barometer.