614 KEPORT— 1903. 



6. The Colours of Iodides. By William Ackroyd, F.I.C. 



The general law of the relation of colour to chemical constitution was stated 

 by the author in 1892.^ Briefly it is that in related compounds of the general 

 formula, A.^Bj,, as B increases in weight (either in atomic mass or multiple of 

 atomic mass) there is increase of absorption of light in definite manner, so that the 

 visible effect is progression in the metachromatic scale from the white towards the 

 black end. AVith one colour vision this would appear like a gradual darkening — 

 an aspect of the phenomenon which the author - regards as being presented by 

 X rays in the photographic effects produced by them after passing through equal 

 thicknesses of the members of a series A,.B„. That this generalisation is reasonably 

 fact-embracing is seen when it is stated that there are only about 2-27 per cent, of 

 exceptions in a survey of some 616 correlated inorganic coloured compounds, and 

 many of these exceptions are of a doubtful nature. 



Iodides conform to the law ; the more heavily weighted molecules have colours 

 nearer the black end of the scale, while the lighter ones, on the other hand, come 

 nearer the white end. Thus in vertical series of the periodic classification arsenic 

 triiodide is orange as compared with the red of antimony and bismuth triiodides ; 

 magnesium, zinc, and cadmium iodides are white, while mercuric iodide is yellow 

 or red. In the periodic groups there are forty-one examples of iodides ; only three 

 are apparently unconformable, two of these being doubtful exceptions. 



When there is more than one iodide of the same metal we have again con- 

 formity to rule, thus : — Hg.J^ is olive green, and Hgl., yellow or red. 



The iodides have also a normal colour when compared with the other h.alides 

 of the same radical as in the series AsF.,. AsClj, AsBr^, and Asl,. In the tabula- 

 tion of these relations conformity to the law is seen both in horizontal as well as 

 vertical groups, and 270 colour facts are presented in such a tabulation which give 

 less than 3 per cent, of exceptions. 



Finally the result of recent research shows that the element iodine has also a 

 normal colour among the other liquid and solid halogens ; their absorption increases 

 from fluorine to iodine through the extremes of white to black. 



It is amply apparent, therefore, that in a comparable series of compounds 

 having similar molecular structure as represented by the same general formula we 

 may have colourless or white bodies at one end and coloured substances at the 

 other end. Hence it is contended that Professor H. E. Armstrong's view that 

 colour is an indication of 'quinonoid structure does not hold for iodides as main- 

 tained by Miss I. Smedley," nor for inorganic bodies generally. 



Tables are given illustrating these various observations. 



7. On Essential Oils. By Dr. O. Silberrad. 



The production of essential oils, although of extreme antiquity, has only 

 recently been made the subject of scientific research. The earlier methods of 

 extraction from the plants were exceedingly crude, and it was only in the early 

 part of the nineteenth century that the industry received a new impulse by the 

 introduction of steam distillation for the recovery of these essences. Chemical 

 research has in recent years led to the replacement of the natural oils to some 

 extent by products artificially prepared. As an instance of this, the author's recent 

 discovery that carvone, Gy^^^fi, the active principle of carraway oil, could be 



' Chem. News, 1893, Ixvii. 27. 



- 0)1 Opacity to the Kontgcn Rays. W. Ackroyd and H. B. Knowles, Jovr. Soo. 

 Dyers and Colourists, vol. xii. April 1896. 

 ' Brit. Assoc. Rspoo-t, 1902, ]}. 582. 



