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Transactions of the Royal Society of South Africa. 
changed to deep green ; it should then be sealed up to prevent absorption 
of moisture. The spectrum of this solution of C?-(HS04).^ shows 
a red transmission band going from X 6880 to X 6770, the position of the 
centre, which is not much affected by varying depth of colour, being at 
X 6825. In very strong solutions, however, the two absorption bands 
bounding this transmission come closer together, and the transmission 
goes only from X 6825 to X 6780. The outer of the two absorption bands 
is very broad, going from about X 7350 to X 6880, and appears rather lighter 
at X 7080, as if it really consists of two bands which coalesce in the centre. 
The inner absorption band is narrow, going from X 6770 to X 6660 with 
greatest intensity at x 6730. The transmission band of the ruby spectrum 
i« at about X 6940, i. e. considerably further up the spectrum. 
A second and even more striking example of the constrained chromium 
spectrum is obtained when glacial phosphoric acid is used instead of sul- 
phuric acid. The product is a thick green syrup, which solidifies to a 
" glass " on cooling, and is a solution of Cr(PO.^);:. in HPO.^. Its spectrum 
shows a transmission band going from X 6800 to X 6705, centre X 6755, very 
similar to that seen in the ruby, but about twice as broad when seen with 
an ordinary slit, and also much further down in the red. In this case the 
whole of the transmission is inside the B line of the solar spectrum, whereas 
in the H0SO4 solution the transmission touches the B line, and the ruby 
transmission is wholly beyond the B line. The outer absorption band has 
its centre at X 6840 and extends from X 6890 to X 6800. The inner absorp- 
tion band crosses the C line of the sun almost symmetrically, its centre 
being at X 6590, and its range being from X 6705 to X 6480. There is also 
a third faint band with centre at about X 6310. This band oan also be seen 
in the emerald. It is probable that by adjusting the concentration of 
chromium in this medium (HPO.^) and so altering the depth of the absorp- 
tion bands it would be possible to get a very close imitation of the emerald 
spectrum. The ruby spectrum, on the other hand, has not been reproduced 
except by means of alumina (i. e. in the artificial ruby) ; probably another 
weak acid (e.g. boric) would do if it could be induced to dissolve chromium 
oxide. 
None of the other acids tried gave anything characteristic, viz. concen- 
trated HCl, HNOo, HCIO4, formic, acetic and citric acids, although a very 
faint hazy band at X 6800 was seen in several of these solutions. Most of 
these comparatively anhydrous solutions have beautiful blue or violet shades, 
and change to bluish green on boiling. 
