320 Kiinz — Electvnmcujneiiv Einh^'ion Tlwot'ij of Light. 



Caiiipbrirs ey.periiueiit niii>lit be tested b}' tueiuis of a Roiit- 

 gen bulb ratbcr than by a source of lip;ht. At all events, there 

 are two important facts which show tliat if the phenomena of 

 light can be explained by an emission theory, the neighborino- 

 trains of oscillations issuing from a source of light must be in 

 a very close phase relation. The ])atli diiference between two 

 beams that can interfere with eacii other can amount to over 

 100,000 waves and the light, snsce])tible of interference, may 

 be exceedingly weak. If light or liontgen rays were able to 

 produce mechanical effects, for instance to disintegrate metals, 

 then the velocity of the iiarticles emitted ought to be indepen- 

 dent of the intejisity of the light, and the quantity of disinte- 

 grated product ought to be proportional to the quantity of 

 light. 



§ 5. Photochemistry. 



A great field of optical phenomena, including the photo- 

 graphic processes, are still awaiting explanation. The funda- 

 mental law of the photochemical action could be deduced at 

 once from the electi'omagnetic emission theory of light. Tlie 

 photochemical action is the same when the product of time 

 and intensity of the incident light remains the same. Exactly 

 the same law holds in the photoelectric effect; the number of 

 electrons emitted from a metal plate under the action of light 

 and Rontgen rays depends only on the quantity of light 

 impinging. These laws are of the same type as Faraday's law 

 of electrolysis. And as in the jDhotoelectric effect and in the 

 ionization of gases the number of electrons or ions is only very 

 small, so also is the number of molecules by exposing a photo- 

 graphic plate to light exceedingly small, whereas from the 

 wave theory, where the electromagnetic energy is disti'ibuted 

 uniformly over the wave-front, we should expect that all mole- 

 cules should behave in the same way. 



The velocity of the photochemical reactions does hardly 

 depend upon the temperature ; the ionization of gases also is 

 independent of temperature ; the mechanism of these phenom- 

 ena appears to be of the same nature. 



JSfow the electrical force in a sunbeam is about 10 volts per 

 centimeter and about 0"5 volts per centimeter in the pulse of 

 the Rontgen rays, while the electrical force required to drag 

 an electron out of an atom of gas is about 2*10° volts per centi- 

 meter, corresponding to the work necessary to produce an ion, 

 which is equal to 4,41 0~" ergs. It is as a rule said, that the 

 ionization of gases, the photoelectric effect and some of the 

 photochemical reactions are due to resonance processes ; but it 

 can be shown that it would require several minutes before 

 ultra-violet light could produce an ion in this way and it would 

 require about 200 days of action of Rontgen i-ays before one 



