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THE ULTRA-VIOLET SPARK SPECTRA. 131 
beryllium and scandium, must be as close as that between scandium and 
yttrium. Now Thalén’s spectra of scandinm and yttrium, though both 
totally unlike the spectrum of any other element, have many characters in 
common;! both spectra contain highly characteristic groups of lines in 
. the orange and yellow regions, the lines or bands degrading towards the 
red, and the number of lines which have been measured are no fewer 
than 103 and 90 respectively. From these two spectra, that of beryllium 
is entirely different, as well in the character and grouping as in the 
number of the lines. Ofthe remaining rare earth-metals at present known, 
cerium is a tetrad, didymium is a pentad, and lanthanum a triad; their 
spectra are quite dissimilar from that of beryllium. In consideration of 
these facts it is impossible to classify the spectrum of beryllium along 
with the spectra of the rare earth metals of the triad group. 
Let us now consider the question of the dyad groups. On the assump- 
tion that beryllium has an atomic weight of 9-2, there is no difficulty 
in placing it at the head of the second series of elements in which position 
it stands in the same relation to the sub-groups—magnesium, zinc, 
cadmium, and calcium, strontium, barium—that lithium occupies with re- 
gard to sodium, potassium, rubidium and copper, silver, mercury. 
Its position is also similar to that of boron and of carbon in relation 
to the triad and tetrad: metals. The spectra belonging to magnesium, 
zine, cadmium, have a very definite constitution ; they consist of—l. A 
single line ; 2. A pair of limes; 3. Three to four groups of triplets; 4. 
A quadruple group; and 5. A quintuple group of lines. The intervals 
between the individual lines in the different groupings increases with the 
increase in the atomic weights of the elements. In fact these spectra 
present a considerable addition to the body of evidence in support of the 
view that elements whose atomic weights differ by an approximately 
constant quantity, and whose chemical properties are similar, are truly 
homologous bodies, or in other words are the same kind of matter in 
different states of condensation. Their particles are vibrating in the 
same manner, but with different velocities. 
In the spectra of the metals calcium, strontium, and barium, succes- 
sive pairs of lines are a strong feature, in addition to which there are 
some other groups in the spectrum of barium. The individuals of each 
pair are separated by smaller intervals the more refrangible the lines and 
by longer intervals the higher the atomic weights. It cannot be said 
that the spectrum of beryllium is similar in constitution to either of 
these groups of elements, which it should be if it strictly belonged 
to one of them, There is some slight resemblance in character to the 
spectrum typical of the calcium group, berylliam having two pairs of 
lines, the individuals of the first or less refrangible pair being separated 
by a greater interval than those of the second pair. It is a spectrum 
analogous to that of lithium, having but few lines and no striking re- 
semblance to the elements which follow in the series because it stands at, 
the head of two sub-groups. Hence it has been concluded that beryl- 
lium is the first member of a dyad series to which probably calcium, 
strontium, and barium are more strictly homologous than magnesium, 
zinc, and cadmium. It is to be understood that this is a conclusion 
drawn from one view only, and is open to correction or modification 
' Kongl. Svenska Ahademiens Handlingar, vol. xii. p. 4, also Comptes Rendus 
vol. 91, p. 45. 
K2 
