Page:The American Cyclopædia (1879) Volume VI.djvu/520

 512 ELECTRO-MAGNETISM say what result will be produced when the conditions are known, or in other words, which will not only present to us the relations of known phenomena, but enable us also to pre : diet the occurrence of those which have not been observed. Without hypotheses of this kind no extended and definite progress can be made in science. It should, however, always be borne in mind that they are the provisional expressions of the generalizations of our knowl- edge at a given time, and that we must hold ourselves in readiness to modify or even aban- don them when we meet with facts with which they are decidedly inconsistent. Two hypoth- eses have been proposed to account for the phenomena of electricity : one, that of Du Fay, known by the name of the theory of two fluids, and the other by that of the Franklinian, of one fluid. According to the first, all bodies are pervaded by two elastic fluids, the atoms of each repelling those of the same kind and attracting those of the opposite kind. When the two fluids are together in equal quantities in the same body, they neutralize each other ; but when separated by friction or other means, their attractions and repulsions are manifested by various electrical phenomena. The second hypothesis supposes that all the electrical phe- nomena are produced by the disturbance of one highly elastic fluid, which pervades the earth and all material bodies, and which is able to move with various degrees of facility or not at all through the pores of substances of different kinds of gross matter, which are hence considered either conductors or non- conductors ; that the atoms of this fluid repel each other with a force varying inversely as the square of the distance ; that the atoms of the same fluid attract the atoms of gross mat- ter, or some ingredients in it, with a force varying in accordance with the same law ; and that the atoms of gross matter devoid of elec- tricity tend to repel each other with a force inversely as the square of the distance. When any body has so much electricity combined with it that the self-repulsion of its atoms is just balanced by the attraction of the same atoms for the unsaturated matter, then the body is said to be in its natural state. So long therefore as all portions of space contain their natural share of the fluid, no electrical phe- nomena are exhibited ; but if, by means of friction, chemical action, heat, or other agen- cies, together with the interposition of partial or non-conducting substances, the electricity is accumulated in one portion of space, and ren- dered to the same amount deficient in another, then two classes of phenomena are manifested : 1, those called statical, such as induction and the consequent attraction and repulsion of light bodies, due merely to the accumulation or de- ficiency of the fluid; 2, dynamical, or those which arise from the transfer of the fluid from the place where it is redundant to that where it is deficient. Franklin's claims to philo- sophic genius rest particularly upon his con- ception of this theory of electricity, which, with slight modifications and additions, is still sufficient to express the connection and relation of the multiplicity of facts which have been dis- covered since his day. However different the two theories at first sight may appear, their mathematical expression and the deductions from them do not differ, provided that we adopt the modification of the latter proposed by ^Epinus and Cavendish, that matter devoid of electricity repels matter; an assumption not inconsistent with the attraction of gravita- tion and chemical action, since we may refer even these to the same cause. The theory of Du Fay was generally adopted by German and French savants, because it was first discussed by them in a mathematical form. The theory of Franklin was afterward developed mathe- matically, and with the modifications we have mentioned is, we think, more readily applicable to the facts of the present state of the science than the other. It follows from the theory of Franklin that if electricity be communicated to a sphere of conducting matter, all the fluid will be found at the surface, because each atom repels the other, and the state of equilib- rium will be that of an equal distribution at the circumference ; the atoms are prevented from flying into space by the non-conducting medium of air in which the globe exists. In like manner it follows from an application of the law of attraction inversely as the square of the distance, that when a body has less than its natural share of electricity the deficiency must exist at the surface. In charged con- ductors of elongated forms, the distribution of the fluid will be greater at the two extremities. The phenomena of the Leyden jar are readily deduced, and all the facts connected with it may be anticipated even with numerical ex- actness, by the application of this theory. When a redundancy of electricity is thrown on one side of a pane of glass, the repulsion acting through the glass will drive off a portion of the natural electricity on the other side, the un- saturated matter of which will attract the free electricity thrown on the first side and thus neutralize its repulsive energy ; and in this way an immense amount of electricity can be ac- cumulated in a small space. When the two surfaces are joined by a conducting circuit a discharge takes place with great intensity, be- cause the fluid on the charged side is impelled through the circuit by the repulsion of its own atoms, and because it is attracted to the other side by the unsaturated matter. If an insu- lated conductor in the form of a long cylinder with round ends be brought near a charged conductor, but not within striking distance, the natural electricity of the former will be repelled to the further end ; the end nearer the charged body will be in a state of deficiency of electricity, or negatively electrified, while the further end will be in a state of redundancy, or positively electrified. Between the two ends there will be a point which will be neutral or