698 Dr. G. Johnstone Stoney on 
temperature does not need to be attended to. But, to go to 
the opposite extreme, let us consider the case of a gaseous 
molecule which has escaped from the earth and travels like 
an independent planet through space. Here no interchange 
of energy can take place between the translational movement 
of the molecule and its internal events. Under suitable 
external influences either of them may be made to vary to 
any extent without this affecting the other. The two kinds 
of energy, or if we please so to call them of temperature, 
have become divorced; and intermediate stages between 
these extremes would be found to exist within an atmosphere 
if we could explore it from its bottom to its top. 
Further distinctions have to be made within the two 
principal kinds of temperature. Those which have to be 
taken into account in the present investigation are the 
varieties of radiation temperature. A body, like the sun, 
acting by radiation upon different gases has no one definite 
radiation temperature, but may be at a different radiation 
temperature in regard to each gas. Thus, the sun is hotter 
with regard to the helium of the earth’s atmosphere than 
with regard to its hydrogen. This we know, because the 
radiations from the sun which can affect hydrogen come 
in the form of the rays corresponding to the hydrogen lines 
of the solar spectrum which are dark, while the radiations 
which raise the temperature of helium come through rays 
corresponding to the helium lines, of which the principal one 
within the visible spectrum—the double line D,;—is as bright 
as the neighbouring part of the spectrum. Hence the 
radiation which reaches helium in the outer part of our atmo- 
sphere has the full intensity of radiation from the sun’s 
photosphere. 
Reviewing the whole case, we find that in the stratum of 
the earth’s atmosphere from which helium escapes, the 
opportunities for exchanging energy between the internal 
motions and the translational, instead of occurring to each 
molecule several times per second, may be so infrequent that 
they occur only once in several hours. During all its inter- 
mediate flights the molecule is exposed during the daytime 
to the full glare of radiation as intense as direct radiation 
from the sun’s photosphere. In this way the internal motions 
of the molecule will be kept for some hours excited to 
intense activity, and if during these hours that special kind 
of encounter happens to take place which affords an oppor- 
tunity for an interchange between the internal and transla- 
tional energies, the two encountering molecules will fling 
asunder with what may be described as explosive violence, 

