From all stagnant places, from all the sinks of overcrowded humanity, from fermenting vegetable and from putrefying animal matter, there are constantly arising poisonous exhalations to do their work of destruction. Where death and decay is a law, this must of necessity constantly occur; but the poisonous reek may be diffused, or it may be concentrated, and Nature has provided for this, and ordered the means for rendering the poison harmless. By the agency of electricity,--probably, too, by the influence of light,--the oxygen in the air undergoes a peculiar change, by which it is rendered far more energetic than it is in its ordinary state. This is the condition to which the name of _ozone_ has been applied. Now, this ozone, or this peculiar oxygen, always exists in the air we breathe; but its quantity is subject to great and rapid variations. It is found that when electrical intensity is high the quantity of this principle is great; when the electrical intensity is low, as in the cholera years, the proportion of ozone is relatively low. This remarkable chemical agent possesses the power of instantly combining with organic matter,--of removing with singular rapidity all noxious odours; and it would appear to be the most active of all known disinfectants. May we not infer from the facts stated that the pestilence we dread is the result of organic poison, which from a deficiency of ozone,--its natural antidote,--exerts its baneful influences on humanity. This deficiency is due to alterations in the electrical character of the air, possibly dependent upon phenomena taking place in the sun itself, or it may be still more directly influenced by variations in the character of solar light, which we have not yet detected, by which the conditions of the electric power are determined. This may be a line along which it is fair to push enquiry. But such an enquiry must be made in all the purity of the highest inductive philosophy, and speculation must be held firmly in the controlling chains of experiment and observation. In the truths, however, which are known to us, there is so much harmony and consistence that even the melancholy theme links itself--a tragedy--with the Poetry of Science. It has been thought, and much satisfactory evidence has been brought forward to support the idea, that the earth’s magnetism is due to currents of electricity circulating around the globe; as a great natural current from east to west--that, indeed, it has an unvarying reference to the motion of the earth in relation to the sun.[152] These terrestrial currents, as they have without doubt a very important bearing on the structural conditions of the rock-formations and the distribution of minerals, require an attentive consideration; but we must, in the first place, examine, as far as we know, the influences exerted, or supposed to be exerted, by electricity, in its varied forms.