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III.— Ow the Existence of Crystals with different primitive forms and physical 

 properties in the Cavities of Minerals ; tvith additional Observations on the 

 New Fluids in which they occur. By Sir David Brewstek, K.H., LL.D., 



F.R.S., and V.P.B.S., Edin. ^ •' , :, ,/^ 



i 



(Read Feb. 17, 1845.) 



In 1823 and 1826 I communicated to the Society two papers on the nature 

 and properties of two immiscible fluids, which I discovered, in contact with each 

 other, in the cavities of topaz and other minerals. Although the facts contained 

 in these papers were of so extraordinary a nature as to be received with scepti- 

 cism by some, and with ridicule by others, yet I am not aware that, during the 

 twenty years which have elapsed since then* publication, any person has either 

 repeated my obseiwations, or advanced a single step in the same path of inquiry. 

 In shewing to strangers some of the leading phenomena of the two new fluids, 

 my attention has been frequently recalled to the subject ; but it was not till last 

 spring, Avhen I discovered cavities in topaz filled with the most beautiful crystals 

 of various form, that I was induced to undertalie a new investigation of their 

 nature and properties. In this investigation I have examined, with various mag- 

 nifying powers, and both in common and polarised light, more than 900 speci- 

 mens of topaz from Scotland, New Holland, and the Brazils ; and I have had the 

 good fortune to observe many new phenomena connected with mineralogy, che- 

 mistry, and physics, which, in addition to the interest which they may possess as 

 scientific facts, promise to throw a strong light upon the existing theories of 

 ciystallization, and to bring before us some of those recondite operations which 

 had been going on in the primitive rocks of our globe, before the commencement 

 of vegetable or animal life. 



1. Cn the Form and Position of the Strata in which the Cavities lie. 



The cavities which contain the two new fluids, and their accompanying crys- 

 tals, sometimes occur single, and in groups more or less numerous ; but, in general, 

 they exist in millions, occupying extensive strata, which affect the transparency 

 of the mineral, and render it unfit for the use of the jeweller, or even for the 

 cabinet of the collector, who has not learned that it is in the deviations from her 

 ordinaiy laws that Nature often discloses her deepest mysteries. 



Although the strata of cavities sometimes occur, as in artificial salts, in planes 

 parallel to the primary or secondary forms of the crystal, yet they occupy every 



