certain Optical Properties of Chabasic. 169 



even proved to belong to one or other of them, an analysis of 

 the successive layers would still be wanting to demonstrate the 

 absolute reality of the cause above explained. 



Whether or not the explanation of the properties in (jues- 

 tion I have here given, be esteemed correct, there are inquiries 

 of no little interest which spring out of the considerations 

 above suggested. It would be important to know how far the 

 presence of one substance in a compound body in greater or 

 less quantity, the others remaining constant, would affect the 

 optical properties; how far those of the compound maybe 

 affected by those of each of its constituents. In regard to 

 form we might in the case of chabasie reason backwards, and 

 inquire how far the whole of the silica might be rejected with- 

 out change of form; whether K+ AL + 6 H should be iso- 

 morphous with silica. 



I have purposely abstained in the former part of this paper 

 from adverting to the form of alumina, or to the possibility of 

 its having a share in the production of the phaenomena to be 

 explained; but if it be isomorphous with silica, then ought 



K + 6 H to be isomorphous with both and with chabasie, 

 I am not aware of the existence of any hydrated oxide in a 

 crystalline form with six atoms of water; so that we are unable 

 as yet to say how far such an analysis of the forms of crystal- 

 lized substances containing one or more constituents isomor- 

 phous with themselves, may with advantage be pursued. 



l!^ote. — The above paper was communicated to the British 

 Association at the Meeting in Dublin in August 1 835*. I am 

 now enabled to adduce two other observations which render 

 still more probable the explanation above suggested. For the 

 first I am indebted to Sir David Brewster, who informs me 

 that a specimen of chabasie he has examinedy/07« Faroe, hasa 

 uniform doubly refracting structure throughout its whole mass. 

 Now the Faroe chabasie, if that analysed by Arfvredson is to be 

 considered a type of the whole, belongs to that kind which con- 

 tains less silica, and throughout which there being supposed no 

 excess of this latter substance above what belongs to the consti- 

 tution of the mineral, no difference of doubly refracting struc- 

 ture of the kind above adverted to ought to be observable. 



2. Among the interesting observations of Biot on the power 

 of certain liquids to cause the plane of polarization to turn to 

 the right or left t> is the remarkable one, that the oils of lemons 



[• See Lond. and Edinb. Phil- Mug., vol. vii. p. 399.— Edit.] 



[+ Translations of the Memoirs by M. Biot in which these observations 



are detailed, will appear in Part II. and sncceeding parts oC the Scieiitijic 



Memoirt. — En ir . ] 



Third Series. Vol.9. No. .5.3. Sept. 1836. T 



