134 Progress in Science. (January, 
in which a substance has been caused to give a continuous spectrum in place 
of its normal one. We may, therefore, ascertain in many cases whether under 
certain treatment a body has or has not suffered a change in composition, and 
trace such a change step by step. 
Absorption Spectra.—There are in the uranium salts two sorts of absorption 
—one direétly related to their fluorescence, and the consequence of the fact 
that those rays which excite fluorescence must themselves disappear, their 
motion taking that other form; and the other an absorption having no imme- 
diate relation to fluorescence, but representing rays of the spectrum whose 
motions are converted into heat or some other form of force not sensible to 
the eye (Phil. Trans., 1852, p. 520). Absorptions of the first class are best 
studied by dire& observation, combined with a process closely allied to that 
described by Stokes as his third method, which consists in throwing a pure 
spectrum upon a screen of the substance in question, or upon the vertical side 
of a tank containing a solution. With the solid screen, the location of 
general maxima of fluorescence will correspond with maxima of absorption, 
and with the tank the absorption can be dire@tly seen as embodied in dark 
blades or triangular masses of shade running into the tank (as seen from 
above) from the side away from the light. These appearances will often 
indicate the existence and relative intensity of absorptions, whose exact 
locations we can measure by examining the transmitted light directly with the 
spectroscope. The spectra of absorptions not diredtly related to fluorescence 
are, as a rule, best studied by transmitted light. The difference between 
different salts as regards their apsorption-bands is very great; and, while in many 
cases solution has a vast effect upon fluorescence, it sometimes produces but 
little effet upon the absorption-bands. In other instances, however, very 
marked changes occur, and, when these are followed out to their legitimate 
conclusions, they lead to some very remarkable results. Thus, if we examine 
the absorption-spetra of the uranic acetate and the various double acetates, 
we shall find that in the solid state they present great variety in the exact 
location of the bands, but in solution we have exactly the same spectrum for 
all. The conclusion therefore presents itself, that in solution all are reduced 
to the same state, which could, of course, only be by the breaking up of all the 
double salts. Indeed, from this, supported as it is by other observations, we 
do not hesitate to conclude that xo double acetate can exist in solution in water, 
but must break up into its two single salts. Nor do our conclusions stop here, 
but we must reserve others until some of the facts on which they are founded 
have been described. A similar experience leads us to a like conclusion in the 
case of the sulphates, oxychlorides, &c. Attention was drawn to the fact of 
such displacement by one of us in September of last year, but its true 
bearing has only been perceived recently, since a large number of observa- 
tions have been accumulated. A change of character, rather than of position, 
produced by solution in the absorption-spectrum of didymium sulphate, was 
observed by Bunsen in 1866 (see ‘“ Pogg. Ann.,” vol. cxxxviii., p. 100, and 
“Phil. Mag.,” 4th series, vol. xxxii., p. 181). The position of the band of 
uranium nitrate, while unaffected by solution in water, is notably changed by 
other solvents, as the following table will show :— 
Bands. I. 2: 3. 4. 5. 6. 
Glycerin’ 3s) f 3.19 <tet (87:6 96'0 107'8 123'0 136'0 148°6 
Water Witte os 889:8 98'5 108'5 118°7 129°5 142'0 
Alcohol. 9/4 cit? Gi.42i— 99°4 I1I'7 — — 
Hydrochloric ether... — 99°4 110°8 123'7 — — 
Ether 2 Lhe) Se 1000 112°6 1230 — _ 
Aceticiether:i..3..-. *“—:  10zf0 DII'7 128'0 135°4 — 
Effects of Heat.—It was observed by Stokes that canary glass and the 
nitrate of uranium had their fluorescence reduced by heating, and that at a 
temperature much below redness their influence upon light in this respect was 
quite suspended. In the solutions of nitrate of uranium all fluorescence was 
extinguished near 212°F. The substance regained its fluorescence on cooling. 
He also remarked that no such action appeared in fluorescent vegetable solu- 
