Lronarp—On the Quantitative Spectra of Molybdenum, &c. 271 
were employed throughout, the numbered lines from 9 to 25 
serving as a ready means of identifying intermediate lines and 
of checking their measurements. In order to trace the gradual 
extinction of the lines, a solution was prepared containing 1 per 
cent. of the element; this was then diluted to contain 0-1 per cent., 
0:01 per cent., and 0-001 per cent. With the last dilution, 
very few lines appear, and in some cases none at all are seen. 
The last letters of the Greek alphabet have been used as before to 
designate the relative persistencies of the lines. Thus :— 
7, seen with the metal, not in solutions. 
o, seen with strong solutions, not in 1 °/, solutions. 
, 39 99 1 mi 99 bb) oP) 0-1 as 39 
X> 9» ” 0-1 sis ” oH) OD 0:01 he ” 
~, 99 39 0-01 “ie 39 99 39 0-001 we 39 
Ws, 99 ” 0-001 WS ” 
On examining all the quantitative spark-spectra which have been 
so far investigated, it is at once evident that the most persistent 
lines of the metals tend towards the more refrangible part of the 
spectrum. At first sight this might appear to be due to the 
greater sensitiveness of the photographic plate to the ultra-violet 
rays. On further examination of the plates, however, it is seen 
in general that, with 1 per cent. solutions, lines appear in the 
visible rays which are quite as strong as those in the ultra-violet ; 
but on dilution they are much more rapidly extinguished. It 
seems, then, that the occurrence of the residuary lines at the more 
refrangible end of the spectrum cannot be put down to the greater 
sensitiveness of the plate for such rays, but must rather be 
attributed to these lines representing the wave-lengths .of those 
vibrations which are most easily excited within the molecule. 
Furthermore, this affords an explanation as to why the ultra-violet 
rays are more chemically active than those of less refrangibility ; 
for they can more readily set the atoms or molecules into vibration, 
and so induce chemical action. 
MoLyBDENUM. 
The spectrum of ammonium molybdate was photographed in 
both acid and alkaline solutions; the results so produced were 
identical. 
