OP ARTS AND SCIENCES. b» 



thick to show the hyperbolas distinctly ; but, by combining it with a 

 plate of a negative uniaxial crystal (tourmaline), we readily obtained 

 the same familiar modification of the interference figure whicli is 

 produced by a very thin plate of mica, and we were thus able, not 

 only to determine that the character of the double refraction of our 

 yellow plate was negative, but also to fix the position of the plane of the 

 optical axes as just described. 



We have stated that, at a veiy moderate temperature, the ortho- 

 rhombic iodide changes to the hexagonal. It is now important to de- 

 scribe this remarkable phenomenon in detail. The change is not a 

 gradual one ; but suddenly, as soon as the required temperature is 

 reached, a red spot appears, generally at one end of the rhombic plate, 

 and then the red color rapidly spreads through the crystal, so that at 

 any one point the change is instantaneous. Again, the change is 

 attended with no disintegration of the crystal or loss of transparency ; 

 and not only the outline, but also all the minute markings, remain after- 

 wards as sharp as before. Externally, there is simply a change of 

 color ft'om greenish yellow to bright red ; and, by sudden cooling, it is 

 perfectly easy to arrest the change so as to leave one part of the crys- 

 tal red, while the rest remains yellow. The change, however, is 

 attended with an entire alteration of structure ; for the optically biax- 

 ial rhombic plate suddenly becomes absolutely uniaxial. Under the 

 polarizing microscope, this change produces a very striking effect. In 

 the dark field between crossed Nicols, the green rhombic plates show, 

 of course, brilliant colors, whenever the diagonals of the rhomb lie 

 obliquely to the planes of polarization ; but, when the change takes 

 place, a dark shadow suddenly spreads over each crystal, extinguishing 

 this color, and then, on removing the analyzer, the very brilliant red 

 color wliich the crystals have acquired appears. Under the polari- 

 scope, the sudden change from a biaxial to a perfectly uniaxial interfer- 

 ence figure is still more striking. We have represented in Plate II. 

 this remarkable phenomenon, as nearly as possible with a chromo-litho- 

 graph. The colors imitate very nearly, although not exactly, those of 

 the natural crystals. 



It becomes now a very interesting point to ascertain what is the 

 exact temperature at which tliis singular change takes place. For 

 this purpose, we heated on watch-glasses small quantities of the 

 yellow crystals to different regulated temperatures, by means of a 

 small air-bath ; and in each experiment the temperature was maintained 

 constant, at the selected point, for at least fifteen minutes. Thus we 

 observed in one series of successive experiments : — 



