December 27, 1889.] 



SCIENCE. 



443 



other body, develops in the latter an electrical condition, that is 

 to say, it electrifies it ; and the process we call " electric induction," 

 to distinguish it from the transferrence of the electrification by 

 contact, which is called " conduction." In the process called in- 

 duction there are two transformations : in conduction there is sim- 

 ply a transferrence, and no transformation. The experimental fact 

 is this : an electrified body sets up in the ether a stress of such a 

 nature, that, by its re-action upon another body, the latter is brought 

 into a condition similar to that of the first ; that is, it electrifies it. 



//. The Magnetic Field. — A magnet in like manner sets 

 up in the ether a stress that is propagated outwards with the ve- 

 locity of light. The physical character of this stress is such that 

 iron and some other substances upon which it can re-act are 

 thereby rendered magnetic. Their molecules are re-arranged. On 

 the supposition that a piece of iron were suddenly magnetized in 

 any way remote from any magnetizable substance, the magnetic 

 field would spread radially, having a spherical surface. As soon, 

 however, as a piece of magnetizable substance was reached, the 

 re-action of the ether upon it would begin ; and the so-called mag- 

 netic lines of force will now be curves, and the equipotential sur- 

 faces will no longer be spherical. The distortion will depend upon 

 the size, shape, and quality of the second body, as well as upon the 

 strength of the field. 



This process is called " magnetic induction." The magnetic 

 field differs from the electric field in this important particular : the 

 latter has no selective property, but re-acts upon all substances, 

 while the magnetic field re-acts upon iron and a few other sub- 

 stances, and but slightly, if at all, upon most bodies. They are 

 alike, however, in this : their equipotential surfaces are determined 

 by the presence or absence of other bodies. 



A magnet then sets up such a physical condition in the ether, 

 that its re-action upon another body brings the latter into a condi- 

 tion similar to that of the first ; that is, it magnetizes it. 



///. — The Thermal Field. — An isolated hot body becomes 

 cool by a process called radiation. It is explained by saying that 



the atomic and molecular vibratory motions that constitute the 

 heat of the body, set up undulatory motions in the ether. These 

 undulations are propagated with the velocity of light, certain wave- 

 lengths being light. The path of a ray is straight, and is contin- 

 ued indefinitely outwards, to the boundary of the universe if there 

 be a boundary; if not, then to an infinite distance. A hot body 

 has a field, as well as an electrified or a magnetized body. 



Experimentally we know that when these undulatory motions- 

 called rays fall upon other matter, it becomes heated in conse- 

 quence ; and we also know that the energy acquired by the secondi 

 body from the radiations depends rigorously upon the area ex- 

 posed to them. It is customary to say that the intensity of light 

 varies inversely as the square of the distance from the source^ 

 when intensity means energy. This is true, however, only for 

 equal intervals of time ; for if a body at unit distance was exposed 

 to radiations from a constant source for one second, and another 

 similar body at double the distance was exposed for four seconds,, 

 each unit of surface would have received the same amount of light 

 or radiant energy. 



The presence or absence of another body in the thermal fieltf 

 makes no difference in the strength of the field in other directions :: 

 in other words, the absorption of radiant energy of this sort makes- 

 no manner of difference in the direction of other rays that have not 

 been stopped. I am not aware of the existence of any evidence- 

 that a ray of radiant energy of any wave-length is ever deflected 

 from its rectilinear course except by a change in the density of the- 

 medium through which it passes, and not then if the incidence be 

 normal. In this respect the thermal field is entirely unlike the 

 other two fields. In addition to this, let it be remembered that a 

 hot body continues to impart its energy to the ether until its in- 

 come equals its expenditure, according to Prevost's law of ex-- 

 changes : so, if there were but a single hot body in the universe, it 

 would impart its energy to the ether, and approach infinitely near 

 absolute zero ; while an electrified body or a magnet would be 

 perfectly insulated, and, so far as is known, would lose none of 



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