Because they restore the _equilibrium of electricity_ which is essential to the salubrity of the atmosphere; they intermix the _gases of the atmosphere_, by agitation; they _precipitate the vapours_ of the atmosphere, and with the precipitation of vapours, _noxious exhalations_ are taken to the earth, where they become absorbed; they also contribute largely to the formation of _ozone_, which imparts to the air corrective and restorative properties. 630. _What is ozone?_ Ozone is an _atmospheric element_ recently discovered, and respecting which differences of opinion prevail. It is generally supposed to be _oxygen_ in a state _of great strength_, constituting a variety of form or condition. 631. _Why do we know that electricity contributes to the formation of ozone?_ Because careful observations have established the fact that the proportion of _ozone_ in the atmosphere is _relative to the amount of electricity_. 632. _What are the properties of ozone?_ It displays an extraordinary power in the neutralisation of putrefactions, rapidly and thoroughly counteracting noxious exhalations; it is the most powerful of all _disinfectants_. Schonbien, the discoverer of _ozone_, inclines to the opinion that it is a _new chemical element_. Whatever it may be, there can be no doubt that it plays an important part in the economy of nature. Its _absence_ has been marked by pestilential ravages, as in the _cholera_ visitations; and to its _excess_ are attributed epidemics, such as _influenza_. It was found, during the last visitation of cholera, that the _fumigation of houses with sulphur_ had a remarkable efficacy in preventing the spread of the contagion. The combustion of sulphur ozonised the atmosphere; the same result occurs through the emission of _phosphoric vapours_; ozone is also developed by the electricity evolved by the _electrical machine_, and in the greater _electrical phenomena of nature_. The smell imparted to the air during an electric storm is identical with that which occurs in the vicinity of an electrical apparatus--it is a _fresh_ and _sulphurous_ odour. The opinion is gaining ground that the respiration of animals and the combustion of matter are sources of ozone, and that plants produce it when under the influence of the direct rays of the sun. It is also believed to be produced by water, when the sun's rays fall upon it. The most recent opinion respecting ozone is, that it is _electrized oxygen_. The subject is of vast importance, and opens another field of discovery to the pioneers of scientific truth. [Verse: "The voice of thy thunder was in the heaven: the lightnings lightened the world, the earth trembled and shook."--PSALM LXVII.] 633. _What is magnetism?_ _Magnetism_ is _the electricity of the earth_, and is characterised by the circulation of _currents of electricity passing through the earth's surface_.