LETTER XXXIV. NEWTON was led to conclude, that there is a fine fluid of the same nature with air, but. extremely more subtile and elastic, every where dispersed through space, which fluid he called æther. That this æther is much more rare within the dense bodies of the sun, stars, planets, and comets, .than in the empty celestial spaces between them; and in passing from them to great distances, it perpetually grows denser and denser, and thereby causes the gravity of those bodies towards one another, and of their parts towards the bodies; every body striving to go from the denser parts of the æther to the rarer: That, therefore, this earth is every where surrounded by this æther to a very great distance, in consequence of which, the air, and all bodies in it, gravitate towards the earth, and towards each other, agreeably to the appearances at the surface of it: That this æther, likewise, pervades the pores of all bodies, and lies hid in them; and whilst the bodies, with this fluid in them, are undisturbed by any external violence, this fluid from its elastic nature, confirms itself as to its degree of density, to the particular make of the body it is in; that is, it is not so dense in dense bodies, as it is in rare ones. This æther of Newton, (for he candidly owned he knew not what it was) and the elec-. trical fluid, may be therefore supposed at no very considerable distance from each other. The word electrical, indeed, is of too confined a meaning to be a proper epithet for a fluid of such universal activity; electricity meaning no more than the power given by friction, to attract and repel light bodies. Thus amber, which when rubbed, was observed to attract bits of straw, down, and other light bodies, was called electrum. The word æther, is Greek, ap, and derived from abw, to burn; because the ancient philosophers, supposed it to be of the nature of fire. Of this very subtile fluid, and which, like the electrical, is supposed to be sui generis, Aristotle says, Anaxagoras believed the most distant parts of space to be filled, looking upon it as a subtile and active fire. What electric fluid, as well as of elementary heat. The air at all times appears to be electrified; its electricity is constantly positive, and much stronger in frosty than in warm weather; but, it is said by some, to be by no means less in the night, than in the day time. In general, the strongest electricity is observable in thick fogs; and the weakest, when the weather is cloudy, warm, and threatens rain. The more elevated the place, as you have experienced on the Alps, the stronger the electricity. But, is the electricity, which manifests itself at the moment a cloud passes over our heads, belonging properly to that cloud, or does it only serve the office of conductor, which transmits to us the electricity of the higher regions of the atmosphere? To me, says De Saussure, it appears clear, that it serves no other purpose than that of a conductor. M. Volta, that profound and ingenious philosopher, has made it evident, that the electric fluid enters into the composition of all vapours; and that, therefore, when the actual heat of the sun, or that which has been a long time accumulated on our globe, draws up in vapour the water spread on the surface of the earth; those vapours drag along with them, a certain quantity of the electric fluid. To a certain height, these vapours ascend. There, being checked by by the cold of the medium, they condense; while the electric fluid that was combined with them, getting its freedom, accumulates in the highest regions, and there continues, until storms, rains, or other conductors, afford it an opportunity of returning to the earth, whence it had its origin. The electric fluid varies in different places. It is, in general, strongest in situations the most elevated, and insulated. In houses, in streets, in allies, and, in general, in places closed around, it is feeble. * Nevertheless, it is sensible in cities; in the middle of large squares; on the borders of quays; and principally on bridges, where it is often found more forcible than in the open country. The hours, when the electric fluid appears feeblest, are those comprised within the time, when the dew of the evening has completely terminated its fall, and the moment of sun rising. From this period, its intensity augments by slow gradation, and arrives sooner or later, but always before noon, to a maximum; which passed, it seems to decline until the dew beginning to fall, it recommences its ascent, and then arrives at its greatest strength: from this time, it regularly diminishes, never, how De Saussure. however, becoming totally extinct, while the air is in any degree serene. The electric atmosphere is, therefore, subject to a flux and reflux like the sea, which increases and decreases twice in the twenty-four hours; but which is not caused, as I have ever heard, by any attraction of the moon. The moments of its greatest force, follow the rising and the setting of the sun; and those of its greatest weakness, are those which precede the rising and the setting of that luminary. In summer, on a serene day, the electric fluid is less strong than in winter. As to its quality, it is invariably positive, as well in winter as in summer; during the day, during the night; and, in fhort, at all times. when there are no clouds in the heavens; and that the electric fluid, should be less strong in summer than in winter, is thus easily accounted for. The air, during the summer, is hot and dry to a considerable height, and, consequently, resists the illapse of the fluid, accumulated in the upper regions of the atmosphere. This fluid, which appears to me, says De Saussure, to be the result of the union of the element of fire, with some other principle yet unknown to us; but, which though probably analogous to inflammable air, must yet be in com |