Dr Brewster on the separation of 



It is not our intention to examine this ingenious resuk of 

 Mr Levy's. We leave this task to Dr Rose, who will no doubt 

 remove some of the objections at least, which have been brought 

 against the separation of Epistilbite from Heulandite. Our 

 object is to take advantage of the perplexity into which the 

 crystallographer and the chemist are respectively thrown ; — to 

 direct the attention of the -philosophical mineralogist to the di- 

 lemma in which his science is thus placed, and to point out 

 the influence of optical characters, and the singular facility of 

 their application, in extricating a doubtful species from the 

 arena of contending opinions, and establishing it upon the basis 

 of unquestionable physical principles. 



I have long ago shown that Heulandite has two axes of 

 double refraction ; that the principal axis is perpendicular to 

 the planes of most eminent cleavage, and that the two systems 

 of polarized rings are easily seen through a plate bounded by 

 the cleavage planes. 



If we now take a plate of Epistilbite comprehended be- 

 tween its planes of most eminent cleavage, and examine its 

 structure by polarized light, we shall look in vain for the two 

 systems of coloured rings. If the two plates are of equal 

 thickness, it will be seen that the double refraction of the 

 Epistilbite, in directions perpendicular to the cleavage planes, 

 is enormously greater than that of Heulandite, the latter at a 

 certain thickness giving the colours of the third order, while 

 the former gives the white light of fixed polarization, and ex- 

 hibits at its edges many orders of colours. 



This experiment, which can be made by the merest tyro 

 in science, establishes between Heulandite and Epistilbite a dif- 

 ference of structure of such a decided character, as to settle for 

 ever the question at issue. Had we merely shown that the two 



