INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 163 



the line X4713 is well-known. Its behaviour under low pressure is, in the light of 

 these experiments, strictly comparable with that of X4471, and these lines are 

 respectively the second members of the two series. 



We may now take up the consideration of the effects produced by admixture of 

 Hydrogen. There is in this case some quantitative information available' in one 

 direction. For in a previous communication, we discussed the effect produced on the 

 spectrum of Hydrogen by the admixture of heavier gases, such as Helium and, more 

 especially in that communication, Neon. It was found that a transfer of energy 

 occurred in the Hydrogen spectrum under these circumstances from the members of 

 lower to those of higher term number, and that, in the quantitative sense, this transfer, 

 which could be measured very accurately, was considerable. We now consider the 

 other side of the problem of inter-action of two gases, from the point of view of the 

 heavier gas. The series arrangement in Neon being unknown, this could not be 

 discussed previously, but the present data for Helium give a basis for discussion. 



Passing now to the Diffuse series of Helium, as shown on photographs (b) and (c), 

 and in Table XIII. , the effect of a small quantity of Hydrogen is very marked. On 

 a scale which preserves X5876 with intensity 10 in each case, the intensity of X4471 

 is 19'5 in pure Helium, but only 11'2, or only half as great, when a trace of Hydrogen 

 is inserted. Moreover, X4026 falls in intensity from 3'69 to 1'63 or less than half. 

 In fact, it falls even relatively to X4471, so that the result implies a definite energy 

 transfer of considerable amount towards the members of low term number in the 

 series, and more especially towards X5876. This is precisely the converse phenomenon 

 to that found in Hydrogen itself when mixed with a large quantity of Neon or 

 Helium. 



The Sharp series of Helium behaves in the same manner, and to an extent which 

 is nearly equivalent, in the quantitative sense. While X4713 is retained at 10, the 

 higher member of lower term number X7065 is enhanced from 0'58 to 073, in the 

 proportion 3'2. At the same time X4121 falls from 2'4 to 2'1 -a change quite outside 

 the possible limits of experimental error in this mode of measurement. We may 

 therefore state, in general terms, that the effect gf a trace of Hydrogen is to throw 

 the energy in the two series much more completely into members of lower term 

 number, so that each is reduced in intensity relatively to any earlier member. 



A comparison of photographs (6) and (d) indicates the effect of a large admixture 

 of Hydrogen. This is quite different, for the Diffuse series shows at once a tendency 

 for transfer of energy in the opposite direction. For on the equivalent reduced scales, 

 X4472 is enhanced only from 13'1 to 19'5, and X402G is unaltered. The phenomenon 

 is therefore in this case not at all defined as a transfer in increasing amounts to 

 the members of higher term numbers. It is apparently the resultant of a 

 combination of this process with the opposite process, resulting in a direct 

 and special enhancement of X4472 of the same nature as we found with low 

 pressure. 



2 A 2 



