3 o THE POPULAR SCIENCE MONTHLY. 



as the circle, the ellipse, parabola, and hyperbola, may be united by 

 the insensible modifi cations 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 dhTerence 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 re- 

 moved. 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 re- 

 fracted. This curious piece of information has been attained by ex- 

 tending 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, polar- 

 ize 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 



