of Edinburgh, Session 1873 - 74 . 
249 
On Certain Motions observed by Mr Sang in Cavities of Iceland 
Spar. By Professor W. Swan, LL.D. 
I have received from my friend, Mr Edward Sang, a crystal of 
Iceland Spar with a letter dated 1st May, in which he W'rites as 
follows : — 
“ In the accompanying little bit of Iceland spar you will find 
a number of microscopic cavities of various shapes, in which you 
may perceive a small bubble of vapour, which serves to show the 
movement of the enclosed fluid.” 
The glass slide carrying the crystal being placed horizontally 
on the stage of a microscope, if “ you bring a piece of metal, say a 
coin, gradually until its edge come almost into the field of view, you 
will see all the bubbles take the (apparently) opposite sides of their 
cavities : that is to say, the metal repels the fluid. On inclining 
the microscope the bubbles take the tops ” of their cavities, and 
“ you will find that the repulsion exceeds gravity in intensity. I 
have only found this repulsion with metals : oxides and sulphurets 
have no action, and each metal has its own specific repulsion. 
Silver is more active than lead, and, if I mistake not, also than 
gold. Mercury has little or no effect.” 
To-day (5th May) I had no difficulty in verifying Mr Sang’s re- 
sult as to the motion of the vapour bubbles when a coin touching 
the Iceland spar was brought near the fluid cavities, but the ex- 
periments I was thereafter induced to make lead to conclusions in 
some respects differing from those which he has obtained. 
Having placed his specimen of Iceland spar on the stage of an 
excellent Boss’s microscope belonging to the United College, and 
using a one-inch object glass, I saw distinctly the motion of the 
vapour bubbles, when a florin piece taken out of my pocket was 
brought up, touching the surface of the spar so to come into the field 
of view, and nearly to cover the fluid cavity observed. The apparent 
effect was the attraction of the vapour bubble, which always ran to 
the side of the cavity nearest to the edge of the coin. I could dis- 
tinctly mark the tendency which the bubble exhibited to run in 
a direction normal to the edge of the piece of metal. 
Before having tried any experiments, and while meditating on 
Mr Sang’s letter, I could not help concluding that most probably 
