( 166 ) 
ultra-red in liquid hydrogen those bands respond as a general 
rule the range of whose permanent phase does not lie too high. 
So in the ease of Sr Mn Na the yellow-red band p it together 
with a little of the green band «. The region of the permanent 
phase for the band lies about —60° C., or very approximately 
between —100° C. and — 45° C., that for the band «, between 
—100° C. and -}-100 o C. Hence the local temperature of the & centra 
had not in any case risen far above —45° C., nor that of the « centra 
far above — 100° C. The same can be seen even in liquid air. In 
a mixture of carbon dioxide and ether (about —50 s C.) on the other 
hand the “phosphor” responds with a clear yellow light (a band) 
just like the afterglow at ordinary temperature, and then becomes 
dark; in this case therefore the local temperature of the « centra 
has already risen above -f-100° C. From this it can be seen that 
the local temperature attainable with a given intensity of ultra-red 
depends upon the temperature of the material filling the space sur¬ 
rounding the centra. Apparently the centra attain that temperature 
at which the energy communicated to them by the ultra-red is 
equal to the energy given in the same time to the surrounding 
material in the form of molecular motion; an equilibrium is esta¬ 
blished just as in the case of warming any body which in the 
same limes loses an equal quantity of heat. 
In this way it has already been explained in “1909” p. 18 that 
different kinds of centra in the same “phosphor” undergo at the 
same time different increases in temperature, according to the greater 
or less efficiency of their insulation from communicating energy to 
the surrounding material. Except for these internal characteristics, 
the local temperature depends upon the intensity of the ultra-red 
rays, as we saw just now was the case with the temperature of 
the surrounding material i. e. of the “phosphor”. The following will 
serve as an example of the dependence of the phenomena upon the 
two latter circumstances. Ca Bi-“phosphors” prepared specially for 
this purpose, show next the blue a group, whose permanent phase 
ranges not far above -f 200° C., also a green £ band whose 
permanent phase lies in the neighbourhood of -f- 300° C. The 
insulation of the energies of both kinds of centra seems to be prac¬ 
tically the same, for when the ultra-red rays are strong enough, it 
responds first blue, -f- 200° C., and then green, -f 300° C. « 
the intensity of the ultra-red rays is now small enough only 
the blue is seen to respond, and apparently the local temperature 
of -}- 300° C. can now be no longer reached. Now increasing the 
intensity of the exciting rays causes the green to appear. When 
