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 PHYSICS

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PHYSICS

this balance and the proof plane, he was enabled to make detailed experiments on the subject of the dis- tribution of electricity over conductive bodies, no such tests having been previously made. Although Coulomb's experiments placed beyond doubt the elementary laws of electricity and magnetism, it still remained to be established by mathematical analysis how electricity was distributed over the surface of conductive bodies of given shape, and how a piece of soft iron w:is magnetized under given circum- stances. The solution of these problems was attempted by Coulomb and also in 1787 by Hatty (q. v.), but neither of these two savants pushed his tests very far. The establishment of principles which would permit of an analysis of the distribution of electricity on con- ductors, and of magnetism on soft iron, required the genius of Simon-Denis Poisson (1781-1840).

In 1812 Poisson showed how the investigation of the distribution of electricity in equilibrium on con- ductors belonged to the domain of analysis, and he gave a complete solution of this problem in the case of two conductive spheres influencing each other, whether placed at given distances or in contact. Coulomb's ex-periments in connexion with contiguous spheres established the truth of Poisson's theory. In 1824 Poisson established on the subject of hollow conductors limited either interiorly or exteriorly by a spherical cavity, theorems which, in 1828, were ex- tended by George Green (1793-1841) to all kinds of hollow conductors and which Faraday was subse- quently to confirm through experimentation. Be- tween 1813 and 1824 Poisson took up the study of magnetic forces and magnetization by impulsion and, in spite of a few inaccuracies which the future was to correct, the formula; which he established remain at the basis of all the research of which mag- netism has meanwhile been the object. Thanks to Poisson's memoirs, the theory of the forces exercised in inverse ratio to the square of the distance, by annexing the domain of static electricity and mag- netism, markedly enlarged the field which at first included only celestial mechanics. The study of the action of the electric current was to open up to this theory a new and fertile territory.

The discoveries of Aloisio Galvani (1737-98) and Alessandro Volta (1745-1827) enriched physics with the voltaic battery. It would be impossible to enu- merate, even briefly, the researches occasioned by this discovery. All physicists have compared the con- ductor, the seat of a current, to a space in which a fluid circulates. In his works on hydrodynamics Euler had established general formulffi which apply to the motion of all fluids and, imitating Euler's method, Jean-Baptiste-Joseph Fourier (1768-1830) began tlie study of the circulation of heat — then con- sidered a fluid and called calorie — within conductive bodies. The mathematical laws to which he had recourse once more showed the extreme importance of the mathematical methods inaugurated by La- grange and Laplace in the study of universal attrac- tion, and at the same time ex-tended by Poisson to the study of electrostatics. In order to treat mathe- matically of the circulation of electric fluid in the interior of conductive bodies, it sufficed to take up Fourier's analysis almost textually, substituting the word electricity for the word heat, this being done in 1827 by Georg Simon Ohm (1789-1854).

Meanwhile on 21 July, 1820, Hans Christian Oer- sted (1777-1851) had discovered the action of the electric current on the magnetic needle. To this dis- covery Andrd-Marie Ampere (1775-1836) added that of the action exerted over each other by two conduc- tors carrying electric currents and, to the study of electro-dynamic and electro-magnetic forces, he applied a method similar to that used by Newton when studying imiversal attraction. In 1826 Ampere gave the complete theory of all these forces in his

"Mdmoire sur la th^orie mathfimatique des ph6- nomenes electro-dynamiques uniquement deduite de I'exp^rience", a work that can stand the test of comparison with the " Philosophia? naturalis princi- pia mathematica" and not be found wanting.

Not wishing to carry the history of electricity and magnetism beyond this date, we shall content our- selves with making another comparison between the two works we have just mentioned. As Newton's treatise brought about numerous discoveries on the part of his successors. Ampere's memoir gave the initial impetus to researches which have greatly broadened the field of electro-dynamics and electro- magnetism. Michael Faraday (1791-1867), an ex- perimentalist whose activity, skill, and good fortune have perhaps never been equalled, established in 1831 the experimental laws of electro-dynamic and electro-magnetic induction, and, between 1845 and 1847, Franz Ernst Neumann (1798-1895) and Wil- helm Weber (1804-91), by closely following Ampere's method of studying electro-dynamic force, finally established the mathematical theory of these phe- nomena of induction. Michael Faraday was opposed to Newtonian doctrines, and highly disapproved the theory of action at a distance; in fact, when he applied himself to analysing the polarization of insulated media, which he called dielectrics, he hoped to eliminate the hypothesis of such action. Meantime by extending to dielectric bodies the formula that Poisson, Ampere, and Neumann had established for magnets and conductive bodies, James Clerk-Maxwell (1831-79) was enabled to create a new branch of electro-dynamics, and thereby bring to light the long-sought link connecting the sciences of electricity and optics. This wonderful discovery was not one of the least important conquests of the method defined and practised by Newton.

XXVU. Molecular Attraction. — While uni- versal attraction, which varies proportionally as the product of the masses and inversely as the square of the distance, was being established throughout the science of astronomy, and while, thanks to the study of other forces also varying inversely as the square of the distance, electricity and magnetism were being organized, other parts of physics received no less light from another Newtonian hypothesis, namely, the supposition that, between two material particles, there is an attraction distinct from universal attrac- tion and extremely powerful, while the two particles are contiguous, but ceasing to be appreciable as soon as the two masses which it acts iiijon are separated by a sensible distance. Among the phenomena to be explained by such attractions, Newton had already signalized the effect of capillarity in connexion with which Francis Hauksbee (d. 1705) had made inter- esting experiments. In 1718 James Jurin (1684- 1750) tried to follow Newton's idea but without any marked success, and it was Clairaut who, in 1743, showed how hydrostatic methods permitted the application of this idea to the explanation of capillary phenomena. Unforljunately his able reasoning led to no important result, as he had ascribed too great a value to the extent of molecular action.

Chemical action also was one of the actions which Newton made subject to molecular attraction, and John Keill (1671-1721), John Freind (1675-1728), and Pierre-Joseph Macquer (1718-84) believed in the fruitfulness of this Newtonian opinion. The hypothe- sis of molecular attraction proved a great annoyance to a man whose scientific mediocrity had not pre- vented him from acquiring great influence, we mean Georges-Louis-Leclerc de Buffon (1707-88). Inca- pable of understanding that an attraction could be other than inversely proportional to the square of the distance, Buffon enteretl into a discussion of the sub- ject with Clairaut, and fondly imagined that he had triumphed over the modest learning of his opponent.