Intelligence and Miscellaneous Articles, 2.29 



that they should be symmetrically arranged by their interposition 

 amongst the essential materials of the molecular edifice, and so 

 adapted to its structure as to participate in its regular arrangement. 



The question, therefore, was to find colouring principles of suffi- 

 cient tenuity to be, as it were, assimilated by the crystals during 

 their formation, to become distributed almost molecularly in their 

 interior without forming accumulations in any one portion of their 

 substance ; and it was necessary to find salts with a molecular tissue 

 sufficiently loose to form regular and homogeneous crystals in 

 strongly coloured, and consequently very impure mother-liquors, 

 whilst nevertheless their formation was not accompanied by a suffi- 

 ciently energetic elimiaatory process for the total expulsion of all 

 foreign matters ; lastly, even when all these conditions were fulfilled, 

 it still remained doubtful whether the production of polychroism 

 would take place in this medium ; for nothing proves it to be inhe- 

 rent in every kind of coloration, and its effective conditions are 

 absolutely unknown. 



The author now lays the following facts, the principal result of an 

 immense number of experiments, before the Academy. 



A colouring matter, disseminated continuously in the interior of 

 a crystal between its lamin?e of growth, but absolutely foreign to 

 the substance, and capable of spontaneous elimination by simple 

 recrystallization from pure water, may nevertheless communicate a 

 property of polychroism and an energy of absorbent action, equal, if 

 not superior, to those of natural coloured bodies, in which these pro- 

 perties are most distinctly manifested. 



The author exhibited large crystals of nitrate of strontia formed 

 in a tincture of logwood which had been rendered purple by a few 

 drops of ammonia. The crystals thus acquired a colour like that of 

 chrome-alum, and a sufficiently distinct polychroism to exhibit the 

 following j)h3enomena : — 



1. Natural white light produces by transmission under certain 

 incidences a red, and under others a blue or violet colour. 



2. Observed with a doubly refractive prism, these crystals are 

 resolved into two images, one red, the other deep violet, according 

 to the thickness ; and these images change colours by passing 

 through the intermediate shades in proportion as the crystalline 

 plate turns in its own plane. 



3. Two similar transparent plates superposed in a parallel orien- 

 tation permit the passage of a portion of the incident white light as 

 a bundle of pur])le rays ; sujjerpused at right angles, they arrest it 

 like the tourmal.iies, or at all events reduce it to a violet tint of such 

 obscurity that it may be considered as destroyed. 



4. Another phenomenon may also serve, if necessary, as a pal- 

 pable demonstration of the intimate connexion established in this 

 compound medium, between the aljsorption thus artificially produced 

 and the natural birefractive jjroperties. 



From these crystals, perfectly homogeneous laminae, slightly in- 

 clined towards the optical axes, may be separated ; when a lamina 

 of this kind is placed very close to the eye and illuminated by natural 



