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of Edinburgh, Session 1872 - 73 . 
various conditions as to temperature; hence the direction of the 
axis of crystallisation is not absolutely kept, as is obvious on any 
of the cleavage surfaces. Hence the path of the extraordinarily 
refracted ray is devious, and the image blurred ; while the path of 
the ordinary ray, depending only on the homogeneity of the 
substance, is straight. For the purpose, therefore, of viewing any- 
thing in the interior it is proper to eliminate the extraordinary 
light by using a polarising reflector, a Nicohs prism, or something 
equivalent. This blurredness of the extraordinary image is 
common in crystals of other substances, and is due to the very 
same cause. 
When a piece of spar containing a flat-faced cavity is placed 
under the microscope, and a small coin or other bit of metal is 
brought near it, the fluid is observed to take the opposite end of 
the cavity. For convenience a type-space was mounted on the end 
of a wire fixed to a stand so as to be readily brought into position, 
and the same repulsion was observed ; here it seemed obvious that 
the metal and the spar had both the same temperature with the 
room, and thus there was no ground for suspicion that temperature 
had to do with the phenomenon. 
Dr James Hunter, while repeating the trials, observed that a 
coin freshly laid down acted well, but that after some time its 
repulsion was less; he observed the same thing of a recently 
rubbed coin. This led him to suspect the agency of heat, and on 
repeating his trials it became clear that a difference of tempera- 
ture is essential to the exhibition of this repulsion. He also found 
that any substance when warmed possesses the same property, and 
lost no time in communicating to me the result of his observations. 
This led Mr E. Elmslie Sang to suggest the trial of metal cooled 
below the temperature of the room ; and, on returning home from 
the Society’s last meeting, I found that Dr Hunter and my son 
had completed a set of trials showing most clearly that the fluid 
in the cavity moves from a warmer and toward a colder body. 
This may be very well shown by placing a piece of metal heated 
in the hand upon the spar, and so sending the fluid to the farther 
end. On now wetting the metal with ether, so as to cool it, the 
fluid is seen to come to the nearer end of the cavity. 
This discovery by Dr Hunter completely changed the line of 
VOL. VIII. 
