Fig. 1 which gives a part of the spectrum of the double sulphate 
of potassium and uranyl at temperatures of 288° K (ordinary tempe¬ 
rature), 80° K. (liquid nitrogen) and 20° K. (liquid hydrogen), shows 
this very clearly. The group in the bluish green is very weak, but 
three groups of bands in the green are very strongly developed. The I 
manner in which the broad, diffuse patches at ordinary temperature 9 
are resolved at the lower temperatures into groups of bands is here 9 
strikingly shown. Figs. 4 and 5 (obtained with the second spectrum 9 
of the grating) give the details of some groups of bands of the same 9 
salt. Some bands which still remained complex at 80° K. have been 1 
resolved between 80° and 20°K. The spectra a and b of fig. 1 were I 
obtained alongside each other upon the same plate by projecting the m 
image of the phosphorescent salt upon two different parts of the slit m 
in succession. From them we can see that by lowering the temperature 9 
the emission-maxima are displaced towards the smaller wave-lengths. 1 
This has already been observed 1 ), but it was not then shown that 9 
the displacement of the maxima was the result of a change in the 9 
period of vibration; it could also have been the result of an unusual 9 
strengthening of bands which existed already at ordinary temperature I 
but had then a very small intensity. The observation of the spectrum 9 
at temperatures at which the bands are distinctly separated from m 
each other shows that we have here to deal with a displacement a 
resulting from the lowering of the temperature. The following 9 
measurements, obtained by comparison with the iron spectrum, show 9 
a distinct displacement of the bands in the case of the double sulphate fl 
of uranyl and potassium between 80° K and 20° K. 
80° K. 511?48 534*24 559*09 586*31 
20° K. 511.35 534.10 558.89 586.05. 
Between 80° K. and 20° K. the ratio of the displacement to the 9 
temperature difference expressed in degrees is much smaller than j 
between ordinary temperature (288° K.) and 80° K. (It is then from | 
2 to &“*). 
The displacements obtained by further lowering of the temperature | 
from 20° K. to 14° K. are scarcely noticeable (fig. 5 a and b), as 1 
was to be expected by extrapolation paying attention to the small j 
difference in degrees between these temperatures. The order of | 
magnitude is from 0,01 to 0,02 pp. Hence, it is very possible that j 
" tke tanperature is lowered, the bands approach asymptotically a 
limiting position. Their change, therefore, is of the same nature as the j 
c hange of vo lume and, perhaps, as the change of dielectric constant. I 
Henri Becquerel, Compt. Rend, t CXLIV 1907 p. 671. 
