4- 18 Sir D. Brewster on the Action of Heat in changirig the 



was a singular sight to see the system of rings travelling along 

 towards tlie line which bisects the optic axes, like a celestial 

 body passing through the field of a telescope, and changing 

 their form and size as they advanced. The specimen did not 

 permit me to see the two systems unite, and still less to see them 

 open out again in a plane at right angles to the laminae; but 

 from the degree of heat which I used, and which drove off the 

 water of crystallization from part of the specimen, I presume 

 that the complete phamomenon cannot be developed without 

 destroying the constitution of the crystal ; that is, that after the 

 two systems of rings have opened out in a new plane, they will 

 not return by cooling, through their state of union, into their 

 primitive inclination of 60° in the plane of the laminse. 



A property of a similar kind, but perhaps a still more ex- 

 traordinary one, I discovered some years ago, subsequent to 

 Professor Mitscherlich's discovery; and I have slightly noticed 

 it in a paper on Glauberite, published in the Edinburgh 

 Transactions*. This interesting mineral has at ordinary tem- 

 peratures the curious property of /too axes of double refraction 

 for red light, and only one axis for violet light. If we apply 

 heat to it, the two optic axes for red light gradually close, and, 

 at a temperature which the hand can endure, the two systems 

 of rings for red light have united into one system, so that the 

 crystal has now only one axis of double refraction for red 

 light. By continuing to increase the heat the two axes se- 

 parated, and the single system of rings opened out into two 

 systems lying in a plane at right angles to that in which they 

 were placed at first. The heat was now less than that of 

 boiling water. By increasing it, the inclination of the optic 

 axes gradually increased. 



I now applied artificial cold to a crystal of glauberite at 

 ihe ordinary temperature of the atmosphere. The inclination 

 of the optic axes for red light increased, as might have been 

 predicted ; but, what was very unexpected, a iicw axis "was 

 created for violet light, the plane of the two violet axes being 

 coincident with the plane of the two red optic axes at and 

 below the ordinary temperature. An increase of cold in- 

 creased the inclination of the optic axes for all the colours of 

 the spectrum ; the inclination of the axes being least for the 

 most refrangible, and greatest for the least refrangible rays. 



These results appear very complicated when we begin with 

 the effects at an ordinary temperature, and view them in the 

 manner in which they were observed ; but if we commence 

 the experiments at a low temperature, such as the freezing 



• Edinb. Pliil. Trans, vol. xi. Part ii. p. 273. 



