722 
MR. W. CROOKES ON RADIANT MATTER SPECTROSCOPY. 
presence of samarium and yttrium ; but it might be argued, from the anomalies that 
arise when both these elements occur together, that in reality the Radiant Matter test, 
however delicate, is one not to be depended upon. For instance, it might reasonably 
be asked what inference is to be drawn in the case of certain minerals treated with 
sulphuric acid and tested in the vacuum tube, and found to give only a feeble 
spectrum ? Does this prove the absence of all but traces of either samarium or 
yttrium, or does it show the presence of both these earths in considerable quantity ? 
The answer is simple. In spite of the perplexing anomalies that have come to light, 
and are described in this paper, regarding the influence of these two phosphorescing 
earths on each other, no single instance has occurred during the work connected with 
this subject in which, with the experience now gained, brilliant phosphorescence and 
a characteristic spectrum could not be evolved from any mixture containing both or 
either of the earths samaria and yttria. If, after treatment with sulphuric acid and 
ignition, the earthy mixture gives a pure spectrum of either yttria or samaria, 
and the line 2693 is absent, it is pretty safe to assume that the particular earth 
indicated is alone present. If, however, the spectrum is not quite characteristic, 
or the anomalous line 2693 is present, it is not sufficient to test the unknown mineral 
or mixture direct in the vacuum tube. It must first be treated chemically to separate 
the samaria and yttria (123, 133), and lime must be added before testing in the 
radiant matter tube (138), when the spectrum immediately makes its appearance if 
either of these earths be present in the smallest quantity. Although I say lime is to 
be added, many other substances perform the same office of eliciting the spectrum, 
such as baryta, lead, &c. (142—145) ; but my chief experience has been with lime, 
and I have always found it to give uniform results under varied conditions. 
One important lesson taught by the many anomalies unearthed in these researches 
is, that inferences drawn from spectrum analysis per se are liable to grave doubt, 
unless at every step the spectroscopist goes hand in hand with the chemist. Spectro¬ 
scopy may give valuable indications, but chemistry must after all be the court of final 
appeal. 
168. Chemistry, however, by itself would have been helpless to solve the difficulties 
had it not been possible to appeal at every step to the radiant matter tube and to the 
spectroscope. The problems to be solved are so new as to be entirely outside the 
experience of laboratory work. A double orange-coloured band shows itself in a faint 
emission spectrum obtained under novel circumstances. On further examination the 
band, or one not far from it, is seen to occur in minerals of very divergent kinds and 
apparently irrespective of their chemical constitution or locality, as well as in labora¬ 
tory reagents and chemicals of assured purity. This band is sometimes accompanied 
by bands in other parts of the spectrum, and occasionally shifts its place to the right 
or to the left. Frequently the orange band disappears and a citron-coloured band 
takes its place. Chemical research continued for a longer time than most chemical 
researches require fails to throw any light on the subject. These being the conditions 
