4,26 Mr. W. Ackroyd on Selective Absorption. 



molar expansion), as, e. (/., in the conversion of ice into 

 water, and water into steam, work ivhich is accompanied 

 by a change of density. 

 0. The overcoming of chemical attraction (atomic reces- 

 sion or molecular expansion), which finally ends in de- 

 composition, as, e. g. y in the resolution of PtCl 4 into 

 PtCl 2 and Cl 2 . 

 I. Now metachromatism is not confined to the coloured 

 bodies one meets with in the laboratory ; for many anhydrous 

 silicates exhibit colour-change, and in some the change is 

 permanent after the metachrome has been subjected to a high 

 temperature. This is shown in the following Table* in each 

 example save that of olivine. 



Density before After ignition, 



ignition. ° 



1. Olivine .... Pistachio -green . . 3-389 Pist.-green . . 3-378 



2. Bervl Straw-yellow .... 2-697 Blue 2-697 



3. Topaz .... Sherry 3-539 Pink 3-533 



4. Zircon .... Brown 4-515 White 4-540 



5. „ Aurora-red 4-863 Colourless . . 4-863 



If we might assume that tivo bodies of identical chemical com- 

 position and density have at a common temperature the same 

 amount of kinetic energy, then it is evident that we may eli- 

 minate kinetic energy from the list of possible causes. For, to 

 take the beryl (2 ), we have differences of colour (that is, of 

 structural absorption) at a common temperature, and the same 

 in the zircon 5, without alterations of density or chemical 

 composition. 



The behaviour of mercuric iodide is perhaps a less equivocal 

 demonstration of this point. Examined spectroscopically at 

 (say) 16° C, a band of red light is transmitted extending 

 from B to D. This narrows as the temperature rises (in other 

 words, there is an increase of structural absorption), up to about 

 140° C. The band of transmitted light now suddenly widens 

 and extends to a little beyond b. Spectra Nos. 5, 6, and 7 

 illustrate this change. After the decrease of absorption as 

 represented by No. 7, we have the normal increase with eleva- 

 tion of temperature; but still at 220° C. there is not so much 

 light absorbed as there was at 16° C. Now the heat or kinetic 

 energy of a body increases with elevation of temperature; 

 therefore the deep orange Hgl 2 at 220° C. has more kinetic 

 energy than the red modification at 16° C. ; but the latter has. 

 greater structural absorption. The rule is that increase of 

 kinetic energy is accompanied by increased structural absorp- 



* Compiled from "Some Experiments on the Density of Garnet, &e." 

 (Church), Chem. Soc. Joura. vol. xvii. pp. 386 & 415. 



