286 Mr Haidinger on the Regular Composition 



of the results of regular composition here described, that I 

 must add a few remarks on this subject, by which it will 

 appear, that, though one of the most common substances, 

 we are sadly in want of an exact knowledge of the forms 

 of crystallized water. Except in the treatise of Mohs, these 

 forms are uniformly described as belonging to the rhombohe- 

 dral system, an opinion rendered highly probable by Dr 

 Brewster's experiments on ice, a substance which shows a re- 

 gular crystalline structure, when examined in polarized light, 

 even in plates several inches thick, and exhibits the single sys- 

 tem of coloured rings, characteristic of the structure of rhom- 

 bohedral and pyramidal forms. There is not, however, in the 

 whole compass of rhombohedral forms one example of a similar 

 formation as the stars with six radii of snow, while it is common 

 enough in those species whose forms belong to the prismatic 

 system. The low degree of temperature has prevented natu- 

 ralists from examining them more closely, the observer him- 

 self being one of the chief causes of their speedy destruction 

 by the action of heat. Their size is often much more con- 

 siderable than that of the crystals of many a species exactly 

 described in our days. On a clear frosty day, when accompany- 

 ing Mr Mohs in a walk from Freiberg to Dresden, I have seen 

 thin six-sided plates of ice nearly half an inch in diameter, which 

 were deposited as hoar-frost on the stalks of reeds in a marshy 

 place, in the forest of Tharand. But from the exertion of the 

 walk, the temperature of our bodies had so much increased, that 

 the delicate flakes would melt on being brought so near the eye 

 as the use of the magnifying lens required. These six- sided 

 plates were striated parallel to their sides ; the reflection of the 

 sun from the broad faces showed a difference of level, in the por- 

 tions t, t, . . u, Fig. 9, seemingly belonging to different indivi- 

 duals. During the intense cold of last winter, I had an opportu- 

 nity of seeing a curious example of crystals of ice, in Berlin in 

 one of Professor Mitscherlich's rooms, which had been the 

 whole winter without a fire. The flowery deposit, formed by 

 a kind of sublimation, on the windows, had considerably in- 

 creased in thickness, and presented all over, when attentively 

 examined, multitudes of crystalline plates, rectangular, and 



