an Electric Source, and Line Spectra. 791 



supplies are derived from the store W. But the last energy 

 equation will hardly be needed in the sequel. This matter 

 has been mentioned only in order to prevent any misunder- 

 standing about the meaning of the store U and of the 

 relative emissivity. 



;' Suggestions as to the possible process of excitation of line spectra. 



For large K the maxima of intensity of radiation J ia J o , &c, in 



ce 2 

 general J; = ~v u i 2 [formula (256), Second Paper], occur at 



u=u x — o^i, u 2 —d 2 u, &c, 



where ${u are given by (36), and, as in (38), are very small 

 fractions. The zeros of radiation intensity are in the immediate 

 neighbourhood of the corresponding maxima, since the}- are 

 attained rigorously for 



U = Mj, M 2 , &C. 



Now, a gas — whose atoms are here identified with similar 

 electric sources — is not luminous in ordinary circumstances. It 

 becomes so, and emits its characteristic spectrum, when it is excited 

 in a peculiar manner, say, by sparks. One might think that the 

 impressed forces e, and therefore the emitted oscillations are 

 produced by the sparks or whatever the external agent might be; 

 that oscillations of all possible frequencies being thus called into 

 existence within the atoms, those and only those whose frequencies 

 are nearly corresponding to «=w,— 5,-u are radiated out with 

 sufficient intensities to be observable as spectral lines. But, in 

 connexion with the explained properties of the electric sources, 

 the following simple and peculiarly fascinating view of the process 

 of exciting spectrum emission naturally suggests itself. 



Let us suppose that in the normal, i. e. non-luminous, state of 

 a gas there are, in the atomic sources, oscillations of rigorously 



&c. 



critical frequencies only *, i. e. e=e Q sin / ^ m J, t = 1, 2, 3, 



Either all of these frequencies may be present in each atom, or 

 some in some atoms, others in other atoms, whose radii are, in the 

 normal state, such that 



na V K 2ira s/ K 



are roots of tanu=u. Then, by what has been shown previously, 

 there will be no trace of emission, whatever the amplitude e Q of 



* As if all other frequencies, just because they have not been critical 

 or strictly internal, were exhausted in the course of ages by continuous 

 radiation, so that out of all possible frequencies only the critical ones 

 could survive. 



\ 



