INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 
103 
the line A47I3 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 (6) and (c), 
and in Table XIII., the effect of a small quantity of Hydrogen is very marked. On 
a scale which preserves A5876 with intensity 10 in each case, the intensity of A4471 
is 19‘5 in pure Helium, but only ir2, or only half as great, when a trace of Hydrogen 
is inserted. Moreover, A4026 falls in intensity from 3’69 to 1'63 —-or less than half. 
In fact, it falls even relatively to A447I, 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 A5876. 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 A4713 is retained at 10, the 
higher member—^of lower term number— A7065 is enhanced from 0'58 to 0’73, in the 
proportion 3'2. At the same time A4121 falls from 2’4 to 2T —-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 of 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, 
A4472 is enhanced only from 13T to 19‘5, and A4026 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 A4472 of the same nature as we found with low 
pressure. 
2 A 2 
