216 Royal Society : — Prof Necker's 



scribed would be greatest when the thickness of the shell was the 

 least : but the inequality thus produced would not, for the smallest 

 thickness of the shell, exceed a quantity of the same order as the 

 polar nutation, and for anj'' but the most inconsiderable thickness of 

 the shell would be entirel}^ inappreciable to observation. 



In his next communication, the author purposes considering the 

 case in which both the solid shell and the inclosed fluid mass are of 

 variable density. 



" Appercu sur une maniere nouvelle d'envisager la theorie cristal- 

 lographique dans le but d'etablir les rapports de celle-ci avec la forme 

 spherique, ou elliptique, des molecules, ainsi qu'avec I'efFet des mi- 

 lieux sur la forme cristalline." Par M. L. A.Necker. Communicated 

 by P. M. Roget, M.D., Sec. R.S. 



In this communication, after adverting to Haiiy's theory of cry- 

 stallization, in which the molecules are considered to be polyhedrons, 

 to the views subsequently taken by WoUaston and Dav}', and par- 

 ticularly to Brewster's conclusions, that there ought to be different 

 forms of molecules, some spherical, some elliptical Avith two 

 equal axes, and a third unequal to these, and others elliptical with 

 three unequal axes ; M. Necker states, that Mr. Dana is the only 

 mineralogist who has attempted to introduce into crystallography 

 the consideration of molecules with curved surfaces. Although, 

 adopting the forms proposed by Brewster, and adding to them those 

 of oblique solids, by introducing the idea of polarity in the axes of 

 crystallization, Mr. Dana has successfully applied this molecular 

 theory to crystallography, yet he goes no farther ; and the most im- 

 portant and difficult steps in this branch of physical science still re- 

 main to be made, and many phenomena in crj'^stallization, with the 

 cause of which we are at present wholly unacquainted, still require 

 to be explained by the theory." The author particularly refers to 

 the important facts discovered by MM. Leblanc and Beudant, of 

 the influence that solutions or mediums in which bodies crystallize 

 have on the secondary forms which these bodies take ; and states, 

 that the present views of crystallography afford not even a glimpse 

 of the least relation between such forms and the j^roperties of the 

 mediums. Why, he asks, does pure water appear, in general, to 

 tend to simplify the forms, precisely as do certain mixtures, those of 

 chlorite in axinite, quartz, felspar, &c., and why, on the contrary, 

 do other mediums, acid or earthy, complicate them ? 



Impressed with the importance which must attach to the solution 

 of such questions, M. Necker offers some ideas which long medita- 

 tion on this important subject has suggested to him. 



Adopting the ellipsoid as the form of the molecule, he remarks, 

 that the more complicated the form of the crystal, the more the 

 number of its faces increases, and the more, at the same time, does 

 it approach to the ellipsoidal form of the molecule ; and, on the con- 

 trary, the simpler the form becomes, the more does it recede from 

 that with a cur^^ed surface. All crystalline forms may be considered 

 as making a part of one or more series, which, in each system of 

 crystallization, have for extreme terms, on the one side, the most sim- 



