in regard to Discoveries in Crystallography. 289 
to the other, by which Professor Weiss endeavours, to remove 
the difficulty, is no less exceptionable than a transition from one 
species into the other. 
The forms of sulphate of magnesia (prismatic epsom salt) are 
evidently prismatic, as you see from the dimensions given in the 
treatise. The prismatoidal cleavage likewise proves this incon- 
testibly. A single perfect cleavage in the pyramidal system 
must be perpendicular to the axis ; but in the present species, a 
position of that kind would produce forms still more incompati- 
ble with the pyramidal system, Professor Mitscherlich also at- 
tributes the prismatic system to this species. 
The system to which we must refer the forms of Wolfram 
(prismatic Scheelium-ore) is likewise by no means the pyrami- 
dal system, although, if we should give no attention to Nature , 
and to the character of the combinations, Hauy’s data might 
lead us to suppose this to be so. But the measurements, or rather 
the indications of the angles of Haiiy, have, in so many in- 
stances, been found incorrect, that we can no longer attach any 
certainty to their exactness. We shall be the less induced to 
do so, the more we find these angles in direct opposition to other 
important observations, which, indeed, are so very obvious in 
Wolfram. 
I shall not expatiate any more upon certain other subjects of 
Professor Weiss’s letter. I declare, that I consider as quite un- 
intelligible what he says of the polarization of the faces, the 
axes , and the sides of lines in the crystalline structure , fyc. The 
data in nature, to which all this refers, are evident in themselves ; 
and I do not think the science promoted, or such phenomena 
explained, and far less their ^ physical principles demonstrated f 
if they are hid in obscure phrases, which tend only to give full 
scope to imaginary speculations. 
Without the least intention to detract from, or depreciate the 
merits of Professor Weiss, and the originality of his own labours 
and publications, I believe I have sufficiently proved, that I 
have not, nay, that I could not possibly have, borrowed any 
thing from him ; an inference which he draws from an assertion 
which he makes, without knowing the circumstances, which I 
had been the proper person to explain, had he but directly ap- 
plied to me. And thus, I think, I have perfectly cleared myself 
VOL. VIII, NO. 16. APRIL 1828. 
T 
