426 M. G. R. Dahlander on the Form assumed by a Fluid 



which by calculation would afford the following per-centage 

 composition : — 



Copper . . . 88-37 



Arsenic . . . 11*63 



100 00 



closely approximating to the results obtained; the mean of 

 which, if the silver be added to the copper, would give — 



Copper . . . 88-41 

 Arsenic . . . 11*59 



100*00 



It is interesting to note the occurrence of the three compounds 

 of copper with arsenic in Chile, although, as far as I am aware, no 

 two of these are found at the same locality ; but all three occur 

 in veins of the same geological age. Their chemical composition 

 is as follows : — 



Domeyldte, Cu«As {^JjjJ * ' S-86 



100 00 



Ai g odo„ite,c«^A S {£PP- ; ;Jjg 



10000 



Darwinite, Cu«As { A >j# \ \ g.'g 



100*00 



The name Darwinite has been adopted in honour of Darwin, 

 whose admirable geological examination of this part of South 

 America is so well known as to require no comment. 



LVIII. On the Form assumed by a Fluid Shell revolving freely 

 within a Hollow Spheroid. By G. R. Dahlander*. 



LET there be a shell of solid matter whose internal and ex- 

 ternal surfaces are both spheroids having the same axis and 

 centre. Within this suppose there to be a hollow fluid shell 

 having the same centre, but not being in contact with the in- 

 ternal surface of the other ; and suppose it to revolve about the 

 common axis with a uniform velocity : then, under the joint in- 



* Communicated by the Author. 



