.254 Mr. William Phillips on the- 



Corundum. 

 Fig. 3. 



The form of the primitive crystal of corundum is a slightly acute 

 rhomboide. From among the numerous fragments in my possession, 

 two were selected, in the form of the primitive crystal, which, from 

 the unusual splendour of some of their planes, gave reason for sup- 

 posing they might be adapted to the use of the reflecting gonio- 

 meter ; one of these yielded the incidences of 86° 1 8' and 93° 45'. 

 The other one incidence of 94° 3". These angles are given by Haiiy 

 86° 38' and 93° 22', and by the Count de Bournon as 86° and 94°.* 



In searching for . the cause of so great difference in fragments of 

 the same substance, it occurred to me that it arose from the nature 

 of the substance itself, or rather from the peculiar aggregation of its 

 laminae. Though the corundum is one of the hardest substances in 

 nature, it is well known that its laminse may be separated without 

 the application of any violent mechanical force, and in some speci- 

 mens, even with ease. Hence it occurred to me that this must be 

 the consequence of some foreign substance being interposed between 

 the layers of the substance itself; and that, if this were the case, it 

 could not be expected to give coinciding measurements, because of 

 the doubt whether the interposed body could be disposed with perfect 

 regularity. There seemed therefore no hope of attaining the desired 

 object, unless the specimen could be reduced into laminse so ex- 

 tremely thin that there should remain nothing but corundum. 

 Finding that this was not to be expected from the common varieties, 

 I sought, and fortunately found a small fragment, nearly colourless 



* Phil. Trans. 1802. 



