292 ON THE OPTICAL PROPERTIES 



easy to submit this point to direct experiment, on account of 

 the difficulty of procuring a mass of agate, from which a varie- 

 ty of transparent prisms could be obtained. It follows, how- 

 ever, from the theory of the depolarisation of light, which I 

 have explained in another place, and which is supported by all 

 the evidence which any theory can possess, that the specimens 

 of agate which depolarise light must necessarily form two di- 

 stinct images, — a phenomenon to which we have found a rapid 

 approximation in the carbonate of barytes. 



The property which has now been explained, forms a simple 

 and infallible mineralogical character of the striated carbonate 

 of barytes ; and is particularly valuable to those who have been 

 perplexed by the numerous marks with which some writers 

 have laboured to distinguish it from its kindred minerals. The 

 assistance, indeed, which optics affords in discriminating mi- 

 nerals, is of the most extensive kind ; and it is much to be 

 wished, that mineralogists would exchange many of their vague 

 distinctions for those unambiguous characters which bodies ex- 

 hibit in the modifications they impress upon light. 



The Abbe Hauy has, in some measure, begun this reforma- 

 tion, and has set a brilliant example of what may be effected 

 by the aid of mathematical and physical acquirements. In his 

 admirable work on Crystallography, which has never been duly 

 appreciated in this country, he has created a new science, in 

 which he has shewn how to determine the integrant molecules 

 of crystallised bodies ; and how, from a few primitive forms, 

 may be derived that endless variety of secondary crystals which 

 adorn the mineral kingdom. The recent discoveries which 

 have been made in optics, enable us to give a new direction to 

 these interesting inquiries ; to determine the forms, and even 

 the angles of crystals, from their optical properties ; and out of 

 a mass x>f shapeless fragments, to reconstruct an artificial crys- 

 tal, 



