610 PEOFESSOE STOKES ON THE LONG SPECTEUM OF ELECTEIC LIGHT. 
opacity for the more refrangible rays. The last, indeed, when dissolved in dilute 
sulphuric acid, is, for this class of bodies, remarkably transparent ; while when the base 
is set free the solution, contrary to what takes place with the other bases, becomes much 
more opaque, but the absorption is vague. I am not sure, however, how far the purity of 
the specimen examined may be trusted, though it was white, and regularly crystallized. 
It would be easy to examine more such substances ; but what precedes is sufficient to 
show the value of the study of the absorption of the rays of high refrangibility, as afford- 
ing distinctive characters of substances little kno\m. 
Minerals. 
I have examined a large number of minerals by the rays from the induction spark, 
both as to their transparency and as to their fluorescence. The transparency of those 
crystals which were of such a form as to permit it, was examined by holding them in 
front of a pure spectrum formed on a fluorescent screen. The fluorescence was sought 
for by forming an image of the spark, for which aluminium electrodes were employed, 
by the 2'5-inch lens, holding the mineral first at the focus of the visible rays, and then 
moving it up towards the lens, and watching for any image which might be formed by the 
rays of higher refrangibility. Should such be observed, its nature was further demon- 
strated by interposing in the path of the rays a very thin piece of mica. This cut off the 
image by intercepting the invisible rays, with respect to which, except a small portion of 
the lowest refrangibility, mica is intensely opaque. 
Carbonate of lime, the sulphates of lime, baryta, and strontia, and colourless fluor- 
spar, were found transparent (sulphate of strontia less so), at least in the qualified sense 
above mentioned, thus demonstrating the transparency of carbonic, sulphuric, and 
probably hydrofluoric acid, and of the bases, lime, baryta, strontia. But this subject 
would be better followed out by salts artificially prepared, and has been investigated by 
Dr. Millee. In two cases results of considerable interest were obtained with reference 
to fluorescence. 
At the time of writing my first paper, on the change of refrangibility of light, I had 
found but one mineral, yellow uranite, to the essential constituents of which the property 
of fluorescence plainly belongs *. In many other cases, both before and since that time, 
I have observed with solar light fluorescence in minerals, but always apparently having 
reference to unknown impurities, and therefore to my mind of much inferior interest. 
By means of the induction spark, employed as above described, I have found one more 
fluorescent mineral f. 
On receiving the image on adularia, and focusing it for the rays of highest refrangi- 
* Philosophical Transactions for 1852, p. 524. 
t The method by which M. Edmond Becquerel has examined the fluorescence of minerals (Annales 
de Chimie, ser. iii. tom. Ivii. p. 43) does not permit of distinct vision of the specimen from the distance of a 
few inches, which seems to me necessary to allow the observer to judge whether the fluorescence which may 
be observed is due to the essential constituents of the crystal or to accidental impurities. 
