The presence of ozone in any vessel or in the atmosphere, may be detected by a test-paper which has been moistened with a solution composed of 1 part of pure iodide of potassium, 10 parts of starch, and 100 parts of water, boiled together for a few moments. Paper so prepared turns immediately blue when exposed to the action of ozone, the tint being lighter or darker according to the quantity. Schoenbein's ozonometer consists of 750 slips of dry bibulous paper prepared in the manner described; and with a scale of tints and instructions, sufficient to make observations on the ozone of the atmosphere twice a day for a year. After exposure to the ozone, they require to be moistened to bring out the colour.
M. Schoenbein continues: 'We must admit that the electric discharges which take place incessantly in different parts of the atmosphere, and causing therein a formation of ozone, purify the air by this means of organic, or, more generally, oxidizable miasma; and that they have thus the important office of maintaining it in a state of purity suitable to animal life. By means of atmospheric electricity, and, indirectly, nature thus attains on a great scale the object that we sometimes seek to accomplish in a limited space by fumigations with chlorine.
'Here, as in many other cases, we see nature effecting two different objects at one stroke. For if the oxidizable miasma are destroyed by atmospheric ozone, they, in turn, cause the latter to disappear, and we have seen that it is itself a miasm. This is doubtless the reason why ozone does not accumulate in the atmosphere in greater proportion than the oxidizable miasma, notwithstanding the constant formation of one and the other.
'In all times, the idea has been held, that storms purify the air, and I do not think that this opinion is ill-founded. We know, in fact, that storms give rise to a more abundant production of ozone. It is possible, and even probable, that sometimes, in particular localities, there may not be a just relation between the ozone and the oxidizable miasma in the air, and that the latter cannot be completely destroyed. Hence, in accordance with the chemical nature and physiological influence of these miasma, they would exert a marked action on the animal economy, and cause diseases among the greater number of those who breathe the infected air. But numerous experiments prove that, as a rule, the air contains free ozone, though in very variable proportions; from which we may conclude that no oxidizable miasm--sulphuretted hydrogen, for example--can exist in such an atmosphere, any more than it could exist in air containing but a trace of chlorine.