preserve the iron from rusting. OZONE. The atmosphere, besides holding the vapour of water diffused throughout its mass, contains also minute traces of carbonic acid and ammonia, and a very remarkable substance called Ozone. Oxygen, one of the component gases of the atmosphere, is capable of existing in two conditions; one in which it is comparatively passive, and another in which it possesses exceptional chemical activity, dependent apparently upon its electrical condition, and in which state it possesses a peculiar smell which has caused it to be named ozone.[16] The characteristic odour is always observable near a powerful electric machine when it is being worked, near a battery used for the decomposition of water, and in the air after the passage of a flash of lightning. Its presence is most marked near the sea-coast, and in localities remarkable for their salubrity; and on account of its influence on health, it has been proposed by Schonbein and others to include ozonometrical observations with the ordinary meteorological observations. [Footnote 16: Greek _ozo_, I smell.] Although in minute quantities it is favourable to health, when existing in undue proportion it irritates the mucous membrane of the nose and throat, producing painful sores. It attacks india-rubber, bleaches indigo, and oxidizes silver and mercury, differing in all these points from ordinary atmospheric oxygen. The chemical energy it possesses (which exceeds that of ordinary oxygen as much as the latter exceeds atmospheric air as an oxidizing agent) affords the means of ascertaining its presence and quantity. It liberates iodine from its combination with potassium, and free iodine colours starch a deep blue. Schonbein, the discoverer of ozone, found that when strips of paper previously saturated with starch and iodide of potassium and dried were exposed freely to the air but protected from rain and the direct action of the sun, they underwent a peculiar discoloration (when immersed in water) after an exposure of 24 hours. A scale of tints numbered from one to ten afforded the means of comparative observation, and thus the Ozonometer was constructed, and a means established of registering the amount of ozone in the air of various localities from day to day. Schonbein also observed that the proportion of ozone was largely augmented after heavy falls of snow. For the exposure of the ozone papers, an ozone cage is employed, as shown at Fig. 67. [Illustration: 67. Ozone Cage. Scale about 1/6.] Ozone may be prepared artificially as a disinfectant by cautiously mixing without friction or concussion equal parts of peroxide of manganese, permanganate of potash, and oxalic acid. For a room containing 1,000 cubic feet, two teaspoonfuls of the powder, placed in a dish and moistened with water occasionally, will develop the ozone and disinfect the surrounding air without producing cough.