428 



Mr. W. Ackroyd on Selective Absorption. 



nexiou between density and colour as due to structural absorp- 

 tion. Not, however, until we have made out a special relation 

 can we well proceed with certainty to establish a comparative 

 one ; nor will this special relation accord with Newton's pre- 

 diction. 



From the nature of metachromatism, it is probable that in- 

 crease or decrease of interatomic distance will in most cases be 

 accompanied by a corresponding alteration of molecular inter- 

 spaces. Hence in allotropes Ave shall look for exactly the op- 

 posite of what Delaval sought to establish, viz. for cold colours 

 with great density and warm ones with low density. 



Metachro- 

 matic scale. 



! 



Carbon, j Sulphur. 



Topaz. 



Titanic 

 oxide. 



Zircon. 



Argentic 

 iodide. 



Calcic 

 fluoride. 



Black 



Brown 



Red 



Graphite, 

 2-35 



Diamond, 

 3-33 



Ductile, 

 1-957 



Octahedral, 

 205 



Pink, 

 3-533 



Yellow, 

 3-539 



Brookite, 

 4-085 



Rutile, 

 4-283 



Brown, 

 4-515 



White, 



4'540 



Dark red, 

 5-406 



Yellowish 

 green, 5 - 681. 



Blue, 

 3-170 

 White, 

 3-176 



Orange 



Yellow 



Blue 



White or 1 

 Colourless J 



The fact of phosphorus being an exception does not militate 

 against this relation, since it is highly probable that the other 

 anomalies of this body will receive elucidation when studied 

 from the metachromatic standpoint. The change in colour of 

 mercuric iodide, which we have already referred to, indicates 

 atomic approach ; and to ascertain whether this is accompanied 

 by a corresponding molecular motion and consequent change of 

 density has been the object of a large number of experiments. 

 From the peculiar difficulties encountered, however, as yet 

 I have obtained no trustworthy results. 



The next step from the special relation we have pointed out 

 to one of a comparative nature would seem to be in an extension 

 of the following law of colour-sequence: — "In a series of anhy- 

 drous binary compounds of the same two elements, those which 

 have the highest amount of the basylous element exhibit the 

 least structural absorption; and, vice versa, those which have the 

 least amount of the basylous element have the most structural 

 absorption"*. 



* Chemical News, vol. xxxiv. p. 76 t 



