126 Proceedings of the Royal Society 
2. Observations and Experiments on the Fluid in the 
Cavities of Calcareous Spar, By Dr James Hunter and 
Edward Sang. 
At a recent meeting I laid before the Society a short notice of a 
phenomenon exhibited by the fluid contained in the cavities of 
calcareous spar. This phenomenon had been observed only a few 
days before, and the notice was given for the purpose of directing 
to it the attention of other observers, and particularly of those who 
happen to possess other minerals with analogous cavities, and I now 
propose to give an account of some .more recent experiments and 
observations in regard to it. 
Of all known minerals, carbonate of lime presents the greatest 
facility for the study of the laws of crystallisation. We trace in it 
evidences of the stoppage and resumption of growth ; we see marks 
of abrasion and fracture on surfaces once external but now covered 
over; layers of mud and portions of extraneous bodies are seen 
inclosed ; yet amid all of these interruptions the direction of the 
planes of crystallisation are kept with remarkable persistence. 
The ultimate or outer surface of a piece of Iceland spar of any 
great size presents a rough appearance, caused by the meeting of 
many surfaces of, as it were, smaller crystals; the hollows among 
these had not been filled up when the deposition ceased. If now 
there be an accession of liquid holding lime in solution, and the 
crystallisation be renewed, these hollows may not be filled up from 
the bottom, but may be covered over by the new mineral, leaving 
the spaces full of the mother liquid, so that when the whole mass 
has been cooled a small vacuity is left. Sometimes these cavities 
are very irregular, at other times their surfaces are beautifully flat 
and often obviously parallel to the cleavage planes of the spar. 
Hence, in mounting such specimens for microscopic observation, 
we must be careful not to heat, or at least not to overheat, the spar; 
cold cement is at all times preferable. 
On looking at any object in the interior of a piece of spar we see 
two images, one belonging to the ordinarily, the other to the 
extraordinarily refracted light. Now, in all bits of spar containing 
faults, the crystallisation has been interrupted and carried on in 
