PEOrESSOE STOKES ON THE LONG- SPECTEUM OF ELECTEIC LIGHT. 607 
requisite to employ. Even alcohol, which in the state in which it is to be had is defective 
in transparency, is sufficiently transparent to be employed as a solvent for such substances 
as those under consideration, provided it be used in small thickness only. 
The alkaloids and glucosides which I have examined are almost without exception 
intensely opaque for a portion at least of the invisible rays, absorbing them with an 
energy comparable for the most part to that with which colouring matters (such as 
alizarine, &c.) absorb the visible rays. The mode of absorption also is frequently, I 
might almost say generally, highly characteristic ; so that by this single property they 
might be distinguished one from another. It frequently happens too that the mode 
of absorption decidedly changes according as the solution is acid or alkaline, which 
assists still further in the discrimination. 
In the examination I sometimes employ a small cell with parallel faces of quartz, 
sometimes a wedge-shaped vessel, having its inclined faces also of quartz, but more 
commonly the former. The cell being filled with the solvent, a minute quantity of the 
substance is introduced, and the progress of the absorption is watched as the substance 
gradually dissolves, the fiuid meantime being of course stirred up. In this way it is 
easy to seize the most characteristic phase of the absorption, which may be then regis- 
tered by the pricking instrument. When minima of opacity occur, it is best to seize 
that stage of the absorption at which they are well developed. When no minima occur, 
a greater or less part of the more refrangible region is quickly absorbed, after which the 
absorption creeps on towards the less refrangible side. When once it has become tole-^ 
rably stationary, the limit of the rays transmitted may be marked. It seems desirable 
not to go beyond this point in the absorption, lest some possible impurity in the substance 
examined, which if it had formed the whole of the specimen would have absorbed rays 
of lower refrangibility, should begin to make itself perceived, and its mode of absorption 
should be mistaken for that of the substance professed to be examined. 
All the metallic spectra are discontinuous, which prevents the mode of absorption of 
even a solid or liquid from being observed quite so well as in the solar spectrum, even 
independently of the greater intensity of the latter, and would greatly interfere with the 
observation of narrow bands like those shown by the absorption of certain gases in the 
visible spectrum, and of which chlorous acid gas (Cl O 4 ) shows a splendid system in the 
invisible part of the solar spectrum. Should a general absorption take place in a part 
of the spectrum where previously a bright group of lines was seen, with weaker light for 
some distance on both sides, it is evident that at a certain stage of the absorption the 
bright group would be left isolated, and the effect might be mistaken for a maximum of 
transparency. In doubtful cases of this kind it is requisite to change the electrodes, so 
as to use the spectrum of some other metal ; but practically the difficulty is not so great 
as might be supposed. 
It is desirable to choose a metal which gives a spectrum that is bright and tolerably 
continuous in the region in which the distinctive features of the absorption are most 
likely to occur. For general use in the examination of substances such as here consi- 
MDCCCLXII. 4 o 
