222 MODERN VIEWS ON MATTER. 



luotioM of electrons. It is known, for inst;ine(% that the \iolent accel- 

 eration or retardation of electrons when they (Micounter an obstacle 

 is responsible for the excitation of K/intoen rays. All lioht and all 

 the Hertz wav^es or pulses employed in wireless telej^raphy are due to 

 electric acceleration, and the greater the rate of change of A'elocity 

 the more violent is the radiation emitted. 



The charge may oscillate, as in a Hertz vibrator, or it may revolve, 

 as in a source of ordinary light, such as a sodium flame. In order to 

 emit perceptible radiation by revolving, it must revolve with extreme 

 speed in a very small orbit, so that its rate of curvature or centripetal 

 acceleration may be considerable; for it is on the square of the value 

 of the average acceleration tluit the energy of radiation depends. 



9. All this is of the nature of a definite and certain thesis, but now 

 we are going to apply it to our hypothesis that the atom of matter is 

 either wholly or partially composed of electrons in a state of vigorous 

 motion among themselves. Such revolving or vil)rating electrons are 

 subject to acceleration, either radial or tangential, and must therefore 

 to a greater or less extent necessarily emit radiation; it becomes 

 natural to iiupiire whence comes the energy that is radiated away. 



Now, in ordinary familiar cases it is the irregular agitation of 

 jnolecules which we call "heat"' that is being radiated away; and in 

 that case the result is a mere cooling, or diminution of the molecular 

 agitation, which can readily l)e made up by receipt of similar energy 

 fi'oiu the inclosures or from surrounding bodies; or, if not made up, 

 it can produce the ordinary well-known efi'ects of ''cold." But to the 

 motion of the internal ])ai'tsof an atom the id(^as of heat and tempera- 

 ture do not apply. Vho atom, if it lose energy, must lose what is to 

 it an essential ingredient, and hence this inevita))le radiating power 

 of the constituents of an atom seemed to constitute a difficulty, for it 

 suggested that an atom of matter was not really a permanent and 

 eternal thing, l)ut that it contained within itself the seeds of its own 

 decay and ultimate dissipation into the separate electrons of which it 

 was composed. The process might indeed be exceedingly slow, the 

 radiation loss might be almost imperceptible, but, in so far as an atom 

 is composed of revolving electrons, it is inevitable that radiation of 

 energy nuist go on from it, and that this nuist in the long run have 

 some perceptible degenerative result. 



10. That result has quite recently, I believe, been experimentally 

 discovered, and is a part of the phenomenon known as '* radio-activity." 



So now we come to the uiost remarkable and probably the most inter- 

 esting step of all. 



The phenomenon of spontaneous radio-activity, discovered lirst by 

 Becquerel in uranium and thorium, and greatly extended by the bril- 

 liant chemical researches of M. and Mme. Curie which resulted in the 

 discovery of radium, was at first supposed to consist in the emission of 



