236 Mr. W. Phillips, on the angles of the 



mountable. Of a veiy large number of crystals of the red oxide 

 of copper in my possession, some hundreds of which were se- 

 lected on account of their extraordinary brilliancy, solely with the 

 view of subjecting them to the reflecting goniometer, I have not 

 been able to find a single crystal of which the primitive planes are 

 adapted to its use. 



The attempt to measure the angles formed by the meeting of the 

 primitive planes of the red oxide of copper, may reasonably be 

 supposed to have been made only for the sake of corroborating the 

 results of calculation. These results cannot perhaps be doubted, 

 when it is considered that the primitive crystal of this substance is 

 the regular octohedron, which is often found to pass into the cube ; 

 for the angles of the one being known with certainty, it follows 

 that those of the other may be calculated with precision. This 

 observation will apply to all those substances which have for their 

 primitive crystals, either of those solids, or the rhomboidal dode- 

 cahedron, the regular tetrahedron, or the hexahedral prism. 



But there are many forms of primitive crystals, included under 

 the term parallelopiped (amongst which are those of quartz and the 

 sulphate of barytes,) as well as several varieties of the octohedron, 

 both acute and obtuse, which do not, with the same ease, seem 

 susceptible of equal certainty in the determination of the value of 

 their angles. I am induced to believe that many, if not most, of 

 these, which have already been given by Haiiy, will be found to 

 demand revision. 



As it may be expected that reasonable cause should be shewn 

 for doubting authority so eminent, I shall briefly subjoin the 

 reasons that have principally led me to adopt this opinion. 



The first step towards ascertaining the value of the several angles 

 at which the numerous planes of crystals meet each other, is alto- 



