( 157 ) 
which does not have its origin in an energy previously absorbed 
from outside sources. I intend, however, to repeat this experiment 
with Rb a S0 4 which has been kept in the dark for three months, 
and am also thinking of experimenting with other rubidium salts. 
Until then I will refrain from drawing further conclusions. 
Physics. — “The behaviour of the phosphorescent sulfides of the 
alkaline earths at various temperatures, and particularly at 
very low temperatures .” By Prof. P. Lenard, Prof. H. Kamer¬ 
lingh Onnes and Dr. W. E. Pauli. Communication N°. Ill 
from the Physical Laboratory at Leiden. 
§ 1. For two reasons it seemed to us of importance that the 
behaviour of the “phosphors” 1 ) of the alkaline earths should be 
investigated at the lowest possible temperatures. In the first, place, 
former experiments of Lenard and Klatt *) in which the tem¬ 
perature was lowered to —180° C., had already shown that the 
development of many emission bands in these “phosphors” would 
end only when the temperature was lowered still further, and it 
was an open question if that further development should correspond 
with the views already deduced (“1904”, pp. 666 etseq.) concerning 
the change with temperature in the properties of the phosphores¬ 
cence bands. Temperatures so low as those which may be reached 
with liquid hydrogen seemed to offer particular facilities for clearing 
up this point, for in other phenomena peculiarities which can be 
attributed to the fixing of electrons become prominent just at these 
temperatures *). 
for the sake of brevity we shall in this paper refer to a phosphorescent 
substance as a “phosphor”. 
2 ) P. Lenard and V. Klatt. Ann. d. Phyc. 15, p. 451 et seq. 1904. 
This paper which contains very numerous experimental data, and which has 
already indicated the main points of the_ theory here given concerning the pro¬ 
cesses involved in phosphorescence, will henceforth, for the sake of brevity, 
be referred to as “1904”. 
*) The changes in the increase of electrical conductivity of metals with lowering 
of temperature noticeable at very low temperatures, in correspondence with the 
idea tirst expressed by Kelvin, that the conductivity attains a maximum value at 
very low temperatures (cf. Kamerlingh Onnes and Glav : Comm. N'. 95q 95^ 
These Proc. Sept. ’06, 99, Sept. ’07, 107*, Nov. ’08), were regarded by Kamerlingh 
Onnes (Comm. Physic. Labor. Leiden Suppl. N°. 9 p. tl 1904) following the views 
of P. Lenard, as a consequence of certain places (dynamids) in the atoms having 
the power at very low temperatures of holding electrons which, at higher tem¬ 
peratures, are in free motion. The different conditions of motion of the electrons 
between and around the dynamids were visualised as three states of aggregation 
