218 DR. W. GEOFFREY DUFFIELD ON THE 



the present experiments was 100 volts. The differences are quite possibly due to 

 different amounts of metallic vapour being present in the arc in the two cases.* 



The hypothesis that the vibrating systems consist of pairs of similar particles which 

 under certain conditions dissociate is suggested, but the system must be such that its 

 period of vibration is not radically altered by the dissociation. 



In connection with this phenomenon attention may be called to the anomalous 

 behaviour of unsymmetrically reversed lines in the iron arc, in which the reversed 

 parts are displaced half as much as the non-reversed line. It is possible that the 

 conditions in the outer layers of the arc correspond to those that hold in the core 

 when giving the lower value of the displacement. 



7. CHANGES IN RELATIVE INTENSITY UNDER PRESSURE. 



As the pressure is increased the relative intensities of the lines change. For 

 broadened lines the intensity is an indefinite term, and the energy of the radiating 

 system which gives rise to the line is better represented by the total photographic 

 action upon the film than by the value of its maximum intensity. The energy 

 emitted under pressure by the system responsible for the very faint hazy bands 

 (a, 4022''83 ; d, 40G3'50, Plate 10) is a specially uncertain quantity, because the 

 energy, though small for each element of the band, is spread out over a comparatively 

 large area. 



The lines that have become obliterated under pressure have become so by one ot 

 two processes, they have either remained fairly definite lines whose intensity has 

 gradually diminished with increase of pressure (I, 4480'59 ; o, 4531'04, Plate 11), or 

 they have been dissipated and become faint bands (d, Plate 10) whose intensity is not 

 sufficient under higher pressures to affect the plate. In the first case the vibrating 

 system seems to have gradually sunk to rest, in the second it appears to have suffered 

 some violent disintegration. The obliterated lines belong to either the First or 



* An attempt was made to test this by using a 50 per cent, alloy of silver and copper. With some 

 difficulty a photograph was taken at 70 atmospheres, but only one line, i (A = 4275), could be satisfactorily 

 measured. Although its displacement is rather less than that of the same line when the pure metal is 

 employed, the value was not a simple fraction of the other. 



For the pure metal the displacement was .... 0-783 A.U. (mean of 12 readings), 

 For the alloy the displacement was -650 A.U. ( 24 ). 



The value for the pure metal is rather higher than is indicated by the average displacement per atmosphere 

 over the whole range ; this is 10-5 thousandths of an Angstrom unit, making the calculated value for 

 70 atmospheres = 0-735 A.U. This is also greater than the displacement of the alloy line. It is difficult 

 to measure the displacements at these pressures with great accuracy, but such small evidence as we possess 

 points to the displacement in the alloy being if anything smaller than in the pure metal. The values are, 

 however, not simple multiples of one another, as in the phenomenon under discussion. It is also doubtful 

 if it is the density that is the determining factor. 



