REVIEWS. 
421 
out gradually, until that of the part farthest from the centre consists of one- 
line. 
In thus employing sodium, bright lines are obtained of different lengths, 
without any continuous spectrum ; and as the distance between the poles is 
increased, one line is seen alone and much brighter than the rest, the yellow 
line remaining after the others have disappeared. “ The spectroscope, thus 
armed, is in fact a new instrument.” 
Radiation and absorption phenomena are next described, and some charm- 
ing modes of demonstration, which space alone excludes here, are described. 
The third chapter deals with spectrum photography. Its history is given 
in Mr. Lockyer’s brilliant style, and will well repay reading, especially his 
quaint description of Mr. Rutherford’s telescope “ through which it is im- 
possible to see anything, but through which the minutest star can be photo- 
graphed with the most perfect sharpness.” 
He then describes his own arrangements, especially that of the fivefold 
slit, and the various means of comparing spectra. Here, as in the frontis- 
piece, and in four other cases, we find a striking whole page illustration, the 
reproduction of a photograph, the method of obtaining which does not 
appear to be given, but which seems to be lithographed in a grey pigment on 
black paper. 
Chapter IV. considers Atoms and Molecules spectroscopically. After a 
recapitulation of modern views as to the molecular condition of matter, 
twenty-one propositions are in turn discussed, many of which are of 
the highest importance. A few of these may be noted : (5.) The spectra 
of both elementary and compound bodies vary with varying degrees of heat. 
(7.) There is spectroscopic evidence that matter is continually broken up as 
the temperature rises. Five stages of this process are given : a. Line spectra. 
|S. Fluted spectra, y. Continuous absorption at the blue end. d. The same 
at the red end. e. “Unique” (Complete?) continuous absorption. (12.) 
Particles absorb light of the same wave-length, and greater amplitude. 
(20.) Some vibrations are very closely connected with others, as evidenced 
by repetition of similar groups of lines in different parts of the spectrum. 
For example, — in sodium the longest line, d, is double. So are all the lines. 
In magnesium the longest line is triple. So is that in the violet. Long and 
short lines are treated next, and the spectra of salts follow. In the section 
on dissociation, the chemist, in the separation of elementary bodies, is shown 
to act mainly by means of vibrations. u He finds the world composed of 
molecules in millionfold complexity, combination, and size, and he acts upon 
these molecules by vibrations. For gross molecules he finds in heat most 
that he wants ; but when the molecules are more delicate, then electricity is 
called in.” With the aid of the spectroscope, additional help is afforded in 
the study, both of the quantity and quality of dissociation, and it is found 
to afford precious suggestions regarding change of molecular structure. The 
conclusions come to are : (1.) That a compound body has as definite a spec- 
trum as a simple one ; but while the spectrum of the simple metal consists 
of lines, the number and thickness of some of which increase with molecular 
approach, the spectrum of the compound consists in the main of channelled 
spaces and bands, which increase in like manner. Both spectra have their long 
and short lines. (2.) That the heat required to act on a compound so as to 
