CONTEMPORARY ADVANCES IN PHYSICS 



75 



turn with altered frequency are evidently relatively rare; otherwise 

 they could not have escaped the notice of those who have studied 

 gases. ^^ Even scattering without change of frequency is unusual, un- 

 less the primary light coincides exactly with a spectrum-line of the 

 molecule ; the blue of the sky is conspicuous only because the air is so 

 thick; in the laboratory, light scattered with unshifted wave-length by 

 a gas can be seen only if the gas is dense, the primary light blindingly 

 brilliant, and the eye thoroughly rested. 



But if it had occurred to any physicist to seek for the effect with (say) 

 mercury atoms, by crowding the atoms together into the liquid form, 

 he would certainly have rejected the idea the moment after it flashed 

 across his mind ; indeed it would probably never have flashed ; for as 



Fig, 1- — Sketch of scheme for observing fluorescence and scattering. 

 (After Pringsheim.) 



soon as atoms are forced into such close proximity, their excited states, 

 or those at least with which we are now concerned, simply disappear. 

 Tho. free mercury atom, for instance, has a "4.9-volt" excited state — 

 that is to say, a stationary state into which it will pass over, if being 

 initially in its normal state it receives an acceptable offer of 4.9 equiv- 

 alent volts of energy. Were we to bombard mercury vapor with 6-volt 

 quanta — i.e. with corpuscles of light, each possessing 6 equivalent 

 volts of energy — we might expect some of these to transfer 4.9 equiva- 

 lent volts to atoms which they strike, and rebound as 1.1-volt quanta. 

 But there is no reason to expect anything of the sort with liquid 

 mercury; there is no reason to suppose that the atoms are avid to 

 grasp this particular amount of energy, and plenty of reason to suppose 

 that they are not. The same holds for every other excited state of a 

 free atom, of which the energy-excess over the normal state is smaller 



" Also one would expect to find, in the light scattered from liquids, quanta which 

 have suffered two or more collisions; such have not yet been reported, so far as I know. 



