REPORT ON PHYSICAL OPTICS. 411 



phenomena arising from unequal condensation and rarefaction in 

 the case of uncrystallized bodies unequally heated. These phe- 

 nomena may be studied by applying a bar of hot iron to the edge 

 of a rectangular plate of glass, and placing it in the polarizing 

 apparatus, so that the heated edge may form an angle of 45° 

 with the plane of primitive polarization. At the end of some 

 time the whole surface of the plate is observed to be covered 

 with coloured bands, the parts near the opposite edges having 

 acquired a positive double-refracting structure, and those near 

 the centre a negative one. The effects are reversed when a plate 

 of glass uniformly heated is rapidly cooled at one of its edges ; 

 and all the appearances vanish when the glass acquires the same 

 temperature throughout*. These phenomena may be endlessly 

 varied by varying the form of the glass to which the heat is 

 applied. If now, by any means, the glass be arrested in one of 

 these transient states, it will have acquired a permanent double- 

 refracting structure. This has been accomplished by M. See- 

 beck by raising the glass to a red heat, and then cooling it 

 rapidly at the edges. As the outer parts, which are thus more 

 condensed, assume a fixed form in cooling, the interior parts 

 must accommodate themselves to that form, and therefore retain 

 a state of unequal density. The law of the change of density, 

 and therefore the double-refracting structure, will depend on the 

 external form; and M. Seebeck found, accordingly, that the 

 coloured bands and patches which such bodies display in po- 

 larized light, assume a regular arrangement which varies with 

 the shape of the massf. The laws of these phenomena have 

 been completely analysed by Sir David Brewster ; and he has 

 shown that the colours are those of crystallized plates, the direc- 

 tion of the axes however being different in different parts of the 

 substance. 



As the double-refracting structure is communicated to bodies 

 which do not possess it naturally, by mechanical compression 

 or unequal temperature, — so, by the use of the same means, that 

 structure may be altered in the bodies in which it already resides. 

 Thus Sir David Brewster and M. Biothave found that the double 

 refraction of regular crystals may be altered, and the tints they 

 display made to rise or descend in the scale, by simple pi*essure. 

 But the changes induced by heat are yet more remarkable. 

 Professor Mitscherlich discovered the important fact, that, in 



• Phil. Tram. 18\6. 



\ The experiments of M. Seebeck are recorded in Schweigger's Journal, 1814. 

 The depolarizing property of unannealed glass seems to have been first noticed 

 by M. Arago ; and was afterwards studied by Sir David Brewster in glass which 

 had been melted and cooled in water. — Phil, Trans. 1814. 



