Sir D. Brewster on the Jctio/i of Heat on Glauberite. 4 17 



Among ihe other crystals which M. Ruciberg will doubt- 

 less examine, we trust lie will not omit sulphate of lime and 

 glauberite, on the doubly refracting structure of which, heat 

 produces such extraordinary effects. — D. B. 



LXXI. On the Action of Heat in cha7iging the Nurnber and 

 Nature of the Optical or remltant Axes of Glauberite. By 

 Sir David Brewster, K.H LL.D. F.R.S. V.P.R.S. Ed. 



CEVERAL years ago Prof, Mitscherlich made the beauti- 

 *^ ful discovery, "that the ordinary sulphate of lime or gyp- 

 sum which, at common temperatures, has two optic axes in 

 the plane of its laminae inclined at 60° to each other, under- 

 goes a great change by elevation of tempei'ature ; the axes 

 gradually approaching each other, collapsing into one, and 

 (when yet further heated) actually opening out again in a 

 plane at right ajigles to the laminae." 



Sir John Herschel, in whose words we have described this 

 remarkable experiment, goes on to observe, " This singular 

 result we cite from memory, having in vain searched for the 

 original source of our information ; but it might have been 

 expected, from the low temperature at which the chemical con- 

 stitution of this crystal is subverted by the disengagement of 

 its water, that the changes in its optical relations by heat would 

 be much more striking than in more indestructible bodies. 

 We have not, at this moment, an opportunity of fully verifying 

 the fact; but we observe that the tints developed by a plate 

 of sulphate of lime, now before us, exposed as usual to po- 

 larized light, rise rapidly in the scale when the plate is mo- 

 derately warmed by the heat of a candle held at some distance 

 below it, and sink again when the heat is withdrawn, which, 

 so far as it goes, is in conformity with the result above stated. 

 Mica, on the contrary, similarly treated, undergoes no appa- 

 rent change in the ])()sition of its axes or in the size of its rings, 

 though heated nearly to ignition*." 



In repeating this im|K)rtant experiment, I made use of one of 

 the specimens described in the Phil. Trans, for J 818, in which 

 1 discovered one of the resultant axes of this mineral. It was 

 about 1 1 inch thick in the plane of the laminas, and the system 

 of rings which surrounded this axis was exceedingly minute, 

 with the usual black brush at each end of them. The other 

 system of rings could not be seen in this specimen, owing to 

 the manner in which it was cut. Having brought the crystal 

 to a considerable heat, and exposed it to polarized liglit, it 



* 'J'rt'atisc on Liglit, Encyclop. Metrop. p. r>(58. 

 'J hird Scries. Vol. 1. No. 6. Dec. 1832. 3 II 



