Page:Popular Science Monthly Volume 12.djvu/40

30 as the circle, the ellipse, parabola, and hyperbola, may be united by the insensible modifications of surface afforded by the inclination, more and more, of a plane dividing a cone asunder.

Similarly, in mechanics, the arc of a vibrating pendulum may be gradually enlarged by successive impacts until it becomes a circle. The part of a rotation differs generically from a complete one, yet it may approach infinitely near to it, and with only such a difference as exists between one arc and another slightly shorter.

The works on physics issued during the last century abound with distinctions which close and accurate investigations have since removed. Iron was once thought to be the only substance endowable with magnetism; now, not only all the metals, but all bodies whatever, are proved to present this polar force. In like manner, with respect to heat and electricity, conductors and non-conductors were ranged as two opposite classes; this disposition is still practically useful, since most substances conduct either very well or very ill; but it has given way as a precise statement of truth before the demonstration that all substances may be placed in unbroken order as to conductive power. For, while no material transmits either heat or electricity without some resistance, that resistance is in no case indefinitely great.

The transmission of light is another property which is not now confined within a narrow area; transparency is no longer attributed to a few bodies only—air, glass, and the like—it is extended to matter universally, experiment and reason both warranting the belief that any substance whatever, if reduced to a sufficiently thin film, would be pervious to light. Gold, one of the densest metals, can now be deposited by electricity in such tenuity as to be easily penetrated by the solar ray, and, although science is unable to give us any metal but gold in a translucent state, we know the degrees at which such light as passes through every member of the metallic catalogue is refracted. This curious piece of information has been attained by extending to the cases concerned a law, which, as far as experiment has gone, has been found true—namely, that the angle of a ray polarized by reflection always makes 90° with the angle of a refracted ray. Now, the particular angles at which lead, copper, and the rest, polarize light by reflection being observed, a simple calculation tells us how much deflection a beam may undergo in piercing metallic plates. This is an instance of how Science appropriates territory, one might deem ever to be undiscovered, by availing itself of the relationship of laws binding all things together, and interweaving the known and the unknown.

Chemists have taken their acids and alkalies, that were formerly adjudged as possessing qualities diametrically opposite, and now include them in one catalogue, no two consecutive members of which are much more than distinguishable in character. The same order