OPTIC AXES IN HAEMOTOME AND WOHLEEITE. 
569 
There is then a separation of about 0° 37' between the plane of the red axes and the 
plane of the blue axes ; and this explains the twisted dispersion which shows itself suf- 
ficiently sensibly by the colours exhibited in plates of Harmotome that are sufficiently 
transparent and are normal to the acute bisectrix, when one examines them by aid of the 
polarizing microscope. 
Among the plates of Morvenite which have allowed, as I before said, of my studying 
afresh the action of heat on the separation and the orientation of the optic axes, the 
most perfect has given me in oil for the red rays, and at 24° C., 2H a =87° 2'. 
If one compares this number with those which I have published in my ‘ Manual of 
Mineralogy,’ it will be seen that the optic axes present separations that vary greatly 
according to the specimens. 
The preceding plate, when placed on the heating cell of the microscope in such a 
manner as to show its optic axes in a horizontal plane perpendicular to the plane of 
polarization, permits of our seeing, when one inclines it sufficiently to right or left, the 
greater part of the bar which traverses the central ring of each system. An elevation 
of the temperature produces an approximation of the axes, and the change in form of 
each central ring, which have been described in my c Nouvelles recherches sur les pro- 
prietes optiques des cristaux naturels ou artificiels,’ while the two transverse bars 
have a tendency to an inclination in opposite directions and are displaced by the same 
amount, the one above, the other below the horizontal plane which at first contained 
them. This displacement, which indicates a rotation of the optic axes, is hardly sensible 
between 15° and 80° C., but from above 90° it becomes very evident and augments with 
the temperature. 
A peculiar and somewhat abnormal character is imparted to this change by the circum- 
stance observed in one of my experiments in which the temperature had been elevated 
up to 160° C. In this case the displacement continued on the increase during the cooling 
of the air-bath from 160° to about 108° C. At the moment of maximum displacement 
the angle through which the plane of the axes had turned seemed at least equal if not 
greater than that which I had been enabled to measure in the case of borax. At 92° C. 
the transverse bars approached again the horizontal position, and began to move in the 
direction of their first orientation, but they did not exactly attain that orientation until 
the lapse of about thirty minutes after the thermometer in the air-bath had come back 
to its original reading. In other experiments, where the temperature was maintained 
between 50° and 52° C., it took twelve or fifteen minutes for the displacement of the 
bars to become sensible. In from fifteen to thirty minutes more this displacement 
had attained its maximum. When the lamp was removed, the return movement of the 
bars hardly became visible before ten or twelve minutes had elapsed after the thermo- 
meter had begun to fall, and thirty to forty-five minutes more subsequently elapsed 
before they had exactly returned to the position they had occupied at the beginning 
of the experiment. 
Harmotome is up to the present time the only crystallized substance which exhibits 
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