226 
NATURE 
[APRIL 29, 1915 

fate, and the German syndicate, together with 
the Austrian Welsbach Company, now practically 
meet the world’s demand. What effect recent 
events will have upon this state of affairs remains 
to be seen, but it looks as if great opportunities 
are opening out for English and American techno- 
logists if they will but take them. 
In a short chapter devoted to the radio-actiye 
minerals, reference is made to the recent estima- 
tion of the atomic weight of the lead contained 
in them; if this element is the end-product of the 
thorium. disintegration, the isotope of Soddy, its 
atomic weight should be slightly higher than that 
of ordinary lead; the value found in a recent 
determination 2084, a result surprisingly in 
accord with theory. 
Part ii. deals with the chemistry of the elements, 
and this section, occupying some 130 pages, is 
perhaps the most valuable part of the book. — It 
commences with a discussion of the properties of 
the elements of the yttrium and cerium groups, 
and of the methods for their separation; the 
greatest success appears to follow the method of 
fractional crystallisation, first successfully applied 
to didymium by Welsbach in 1885, when he suc- 
ceeded in separating that supposed element into 
two bodies, having slightly differing atomic 
weights, naming them neo- and praseo-dymium; 
it will be remembered by some that the values 
published at the time by Welsbach were inverted, 
praseodymium being given at 143°6 and _neo- 
dymium as 140°8, and it was not until 1808 that 
Brauner discovered and directed attention to the 
mistake. 
The discussion of the spectrum examination is 
a little disappointing ; the use of the spectroscope 
in the analysis of the rare earths is of immense 
importance; the absorption spectra can now be 
photographed from the extreme ultra-violet down 
to the least refrangible limit of the visible spec- 
trum if a suitable source of light is combined with 
the panchromatic plates that are now on the 
market. 
In dealing with emission spectra it is suggested 
that to observe spark spectra one pole of an induc- 
tion coil is embedded in the oxide to be examined 
and a spark passed! This seems to be a very 
unusual procedure ; in general practice the spark is 
either passed between platinum wires in a strong 
solution of the earth or else carbon rods saturated 
with the solution are used as electrodes; reference 
is made to the ‘‘reversion spectrum” discovered 
by the late Lecogq de Boisbaudran, but no details 
are given; as in some cases this method is of 
considerable value, it may be worth while to note 
that it is described in the Comptes rendus for 
1886 (vol. cii., p. 153). 
NO. 2374, VOL. 95] 
is 
We must take exception to the rather summary 
dismissal of the kathode luminescence spectra dis- 
covered by Sir William Crookes some years ago; 
it is stated that the researches of De Boisbaudran, 
Baur and Mare, and Urbain have shown that’ 
these spectra were due to minute traces of a 
coloured earth in a large quantity of a colourless 
one, and that the sensitiveness of the method is 
so great that it cannot be employed for the ordin- 
ary purpose of chemical analysis; this may be 
partly true, but there is abundant evidence of the 
value of the’ kathode spectra when applied to the 
rare earths if the technique of the application is 
thoroughly understood. The, sulphates of many 
of the earths give discontinuous spectra. of great 
beauty and minute detail quite distinct from the 
nebulous luminosities produced by mixing coloured 
and uncoloured earths and submitting the mixture 
to the action of kathode rays; the oxides also, when 
in a state of purity and under certain conditions, 
give rise to a further novel series of characteris- 
tic phosphorescence spectra, and it is a matter of 
regret that this method of analysis has not re- 
ceived the attention that it deserves. We feel that 
the possibilities connected with it are too valuable 
to allow it to be lightly set aside as worthless. 
For the general examination of the rare earths 
are spectra are stated to be the most trustworthy, 
and a valuable feature is the inclusion of lists of 
the dominant lines in the spectrum of most of the 
elements. 
The last section, the technology of the ele- 
ments, is largely occupied with an account of the 
“mantle” industry; from a very interesting his- 
torical sketch it appears that the employment of 
the rare earths had been suggested for the pro- 
duction of light from very early times; mantles 
of platinum coated with lime and rare earths were 
used so far back as 1839, and mantles made from 
gauze that had been impregnated with lime and 
magnesia were used in 1849. The success tha‘ 
has followed the invention of Auer von Welsbach 
is due in great measure to the accidental dis- 
covery that the combination of about 1 per cent. 
ceria with thoria greatly increases the luminosity 
of the material, and this composition is now 
universal. 
The commercial preparation of thorium from 
monazite is fully set out, and the various fibres 
used for forming the skeleton of oxide are de- 
scribed. Cotton has been replaced by ramie 
thread, and this in turn is giving way to artificial 
silk and similar materials, which on account of 
their uniformity are found to produce more robust 
and lasting skeletons. : 
In addition to the mantle industry there are 
| already many other valuable technical applications 

