524 REPORT—1904. 
est dissous et chauffé avec un fort excés de potasse concentrée, il arrive un 
moment ot la coloration rouge passe au vert; en méme temps il se dégage de 
Yoxygéne trés abondamment et il se dépose des cristaux verts de MnO‘K? qu’on a 
le temps de laver avec de la potasse froide, de l’alcool absolu et de I’éther. 
Ces cristaux verts, dissous dans l’eau et traités par un courant d’air, absorbent 
Voxygéne a froid ; la liqueur devient rouge et il se dépose des cristaux de perman- 
gvanate MnO‘*K. Les deux réactions successives s’écrivent donc: 
(1) 2MnO'K +2KOH =2Mn0‘K?+ H20+0 
(2) 2MnO‘K? + H?0 +0 =2MnO‘K + 2KOH, 
Ce sont ces deux formules qui me paraissent nouvelles, sauferreur. Tlles ex- 
pliquent les apparences souvent controversées du caméléon minéral. L’oxygéne 
qui est dans l’eau aérée oxyde le manganate vert. I] n’y a ni équilibre chimique 
spécial ni influence de l’acide carbonique ni dépdt de bioxyde de manganése. 
9. On the Bearing of the Colour Phenomena presented by Radium 
Compounds. By Witt1am AcKRoyp, 
Madame Curie early observed that radium salts are white like barium salts 
when first prepared, but that they gradually become coloured.’ I have associated 
this colour-change with the reception of external radiant energy and the con- 
sequent increase of radio-activity.” What I take to be confirmation of this view 
has been obtained in following the history of a radium bromide tube. The salt 
was nearly white when purchased, and in a month or two it became tawny-yellow 
or orange; its powers of absorption had visibly increased, and its power of exciting 
fluorescence in barium platino-cyanide was concurrently quadrupled. These facts 
may be thus diagrammatically represented :— 
Colour-change and increase of absorption. 
Seral 
White, yellow, orange, tawny-orange. 
—> 
Increase of radio-activity. 
This readiness on the part of radium compounds to become coloured under the 
influence of external radiations may be regarded as an effort to conform to the 
constitutive-colour law ;* moreover, it is highly probable that the colour-change 
is effected by a minimum expenditure of energy, as in the case of other end 
members of vertical groups of the periodie classification of the elements. This 
tendency I have experimentally demonstrated in the case of chlorides of alkali 
metals.* 
But while we may regard this colour-change as being slowly effected by 
external radiations, the comparatively sudden application of sensible heat has a 
very different effect. In July 1903 I strongly heated tawny-orange radium 
bromide, expecting to see it change in this order—red, brown, black-like mercuric 
oxide and other colour-changing bodies. Instead, the seething substance reverted 
at once to white. The colour behaviour of the radium bromide when heated is 
evidently anomalous like that of some other compounds of extreme members of 
vertical periodic groups; as, for example, mercuric iodide. Its powers of absorption 
are visibly lessened, and, as is now well known, the radio-active properties of the 
residue are also lessened. The increase in the radio-activity of the expelled 
emanation is another phenomenon, and probably the joint produce of radium rays 
and heat. Analogous effects are produced by radium rays and heat on halides of 
1 Thesis on Radio-active Substances, p. 30. London: Chem. News Office. 
2 Lancet, Nov. 21, 1903, p. 1464. 
% B.A, Report, 1903, and Chem. News, 1903, 88, 217. 
4 Journ. Chem. Soc., 1904, 85, 815. 
