| Sept. 10, 1885 | 
nations. In the future we must look more to men and 
to ideas, and trust less to mere systems. Systems have 
had their trial. In particular, systems of examination have 
been tested and found wanting in nearly every civilised 
country on the face of the earth.” 
What we have written will show what food for thought 
in the matter of our present needs has been provided at 
Aberdeen for those gathered together for the advance- 
ment of science. Surely the three addresses to which we 
have specially referred in the present article suggest a 
gap in the organisation of the Association. Why should 
there not be a section to deal specially with the question 
of Education and Research ? 
THE “DECOMPOSITION” OF DIDYMTUIT 
i | NDER the above title the Chemical News has recently 
reprinted from the Chemiker Zeitung a notice of 
an important piece of work recently communicated to the 
Vienna Academy by Dr. C. A. von Welsbach. The work 
appears to have resulted in the discovery that the ‘‘ dyad 
or triad element ” didymium with an “ atomic weight” of 
48 or 96, or 147, according to the text-books employed, 
and which since its separation by Mosander in 1841 has 
been investigated by Marignac, Hermann, Watts, Bunsen, 
Deville, and Erk, not to mention many others, is no 
element at all, but is built up of two substances which 
can be separated from each other by an ordinary chemical 
process. The “decomposition” was in fact effected by 
means of the double ammonium or sodium nitrates in 
presence of lanthanum salt. 
The colours of the salts of the two substances are quite 
different. The salts of that which approaches lanthanum 
in its chemical characteristics are of a leek-green, those 
of the other substance are rose or amethyst red, and it is 
this substance which exists in greatest quantity in didy- 
mium. Dr. von Welsbach proposes for these two new 
substances the names of “ praseodymium” and “neo- 
dymium.” 
It will be readily seen that from the chemical point of 
view alone these results are of very high interest, but 
there is another from which they assume a very great 
importance. 
The “element” didymium after it was separated by 
the chemist had been handed over to the physicists. 
Gladstone, we believe, was among the first to note the 
characteristic absorption spectrum of the salts. In this 
work he was followed by Bahr and Bunsen, Erk and 
others. Thalén determined its spark spectrum, and in 
our spectroscopic literature didymium has taken its place 
by the side of hydrogen and iron as a characteristic 
spectrum-giving element. 
Now one of the arguments which has been used in 
support of the view put forward some time ago of the 
dissociation of the chemical elements at solar tempera- 
tures is that at one “heat level” in the sun’s atmosphere 
(a term coined because the sun’s atmosphere must get 
hotter as we go down, and we have means of determining 
which vapours ascend from hotter regions and which 
descend from cooler ones) we get some lines of the spec- 
trum of a substance, let us say iron, and at another we 
get others ; so that to get the complete spectrum of iron, 
as we see it when we use iron in our laboratories, we have 
NATURE 
455 
to add together the two sets of lines seen in the spectra 
of parts of the sun known to be at different temperatures. 
To make our statements more precise we may say that 
the lines of iron seen bright in the spectra of solar 
prominences and those seen widened in the spectra of 
solar spots are so different that it may be said that there 
is hardly a line common to both. So much so that, as 
was said years ago, if we did not know iron here, and the 
fact that its spectrum contains both sets of lines, we 
should say that the prominences gd iron contained one 
substance, and the spots gw@ iron contained another. 
These facts were explained by the hypothesis that there 
were in the so-called element iron at least two different 
substances or molecular groupings, one of which alone 
could withstand the higher temperature of the promin~- 
ences. The reason that do//% sets of lines and many 
others are seen in the spectrum of iron in the high- 
tension spark is that the temperature of the spark is 
sufficient to carry the solid metal through the series of 
simplifications, whether many or few, which lie between 
the limits formed by the solid state and the temperature 
of the prominences. 
_ To this it has been objected that if these things exist in 
iron they should be isolated and put in bottles. To this 
it has been replied that the bottles themselves must be 
incandescent, or the “things” will unite again as they 
have done before to form iron as we know it. 
Now the real importance of Dr. von Welsbach’s work 
is that what has not yet been done for iron—to prove 
beyond all cavil the above hypothesis—he has done for 
didymium. He has got into two bottles, which we may 
mentally label “spot bottle,” “ prominence bottle,” two 
substances from the “element” didymium, each of which 
has a characteristic spectrum consisting of different parts 
of the spectrum of didymium just as the spots and promin- 
ences have spectra gud iron, which are different parts of 
the spectrum of iron. Further, by mixing the substances 
in these two bottles together in proper proportions he can 
produce a third, which gives the mapped spectrum of 
didymium.exactly as in the general spectrum of the sun, 
in which we get, added together, the absorptions of the 
hotter and cooler regions represented by prominences 
and spots, we have gzd iron, something not unlike the arc 
spectrum of that substance. 
There is no doubt that the interest of both chemists 
and physicists will be keenly excited by Von Welsbach’s 
work, and that it will be critically examined and re- 
peated. If it be confirmed we may hope that some day 
similar work will be undertaken here. The way is open, 
and has been cleared in a remarkable way. Formerly it 
was imagined that very high temperatures and new 
chemical methods were the sole agents to which appeal 
could be made in such a case; it may turn out that 
there are reagents to hand if chemists will turn their 
attention to them. 
It is further clear that the ‘‘elements” with high 
atomic weight should be the first to be attacked. Those 
who consider the spectrum of cerium, for instance, which 
in the blue and violet portion is richer in lines than the 
spectrum of the sun itself, to be produced by the vibra- 
tion of “the chemical atom” or “ the chemical molecule,” 
no matter which, will find themselves in a hopeless 
minority, now that the simpler explanation of a mixed 
