( 169 ) 
at higher temperatures and up to the beginning of red heat, the 
“phosphor” enclosed in a flat quartz tube was placed in a hot-air 
bath and illuminated by a spectrum. From these experiments it 
appeared that those parts of the spectrum to whose excitation mo¬ 
mentary response was given and which we shall call collectively 
the “momentary part”, remained momentary as far as low tempe¬ 
ratures up to those of liquid air x ) were concerned at all temperatures, 
while those portions of the spectrum which induced an afterglow, 
the “permanent part”, induced this afterglow only within the tem¬ 
perature range of the permanent phase for the band in question, as 
was to be expected. At the temperatures of the upper or of the 
momentary phases also the “permanent part” becomes momentary 
in accordance with the properties of this phase. 
It is especially worth noting that whenever the bands are in the 
permanent phase, the particular momentary part overlapping the 
“permanent part” nevertheless remains unchanged, so that when a 
band is excited even at temperatures within the permanent phase, 
the two processes which we have called “momentary” and “perma¬ 
nent” proceed simultaneously *) in such a way that the quantitative 
relation between the two processes differs according to the exciting 
wave-length. Some wave-lengths induce almost the momentary process 
alone, others almost only the permanent process; in general, any 
wave-length will induce both processes at the same time. For instance, 
X — 400“ excites Sr Cu « momentarily, X = 434^" permanently ; 
A = 200w* and X = 450w* excite SrZn« momentarily, X = 366^" per¬ 
manently ; in the same way X = 200“-“ excites CaMna momentarily 
and 1 = 334^ permanently. The general characteristic of the excita¬ 
tion-distribution is that the shortest wave-length (in the extreme ultra¬ 
violet) induces only the momentary process, and that towards the 
limit of the excitation-distribution on the side of the greater wave¬ 
lengths continuous and momentary excitation interchange periodically 
one or more times*). 
If the permanent part in the afterglow is to be well observed, 
illumination must be continued for several minutes with a very 
intense spectrum, for it has always appeared from the experiments 
that a powerful (“1904” p. 477) and also a prolonged (“1904” 
q In liquid hydrogen the observation as already indicated § 6 proved too difficult. 
2) This point has already been elucidated particularly with reference to the 
Sr Gu a band (“1904” p. 668). By special measurements A. Werner has demon¬ 
strated it for the SrZna band. 
3 ) A separate communication will deal with these characteristics. 
12 
Proceedings Royal Acad. Amsterdam. Vol. XII. 
