$819.] "in Reply to Dr. Wollaston. 127 
relative Importance of Crystalline Forms and Chemical Composi- 
tion in the Classification of Mineral Species.” I regret very 
much that I have received it just the evening before my depar- 
ture for a long journey, and that this circumstance prevents me 
from entering into some new details which are due to the high 
consideration in which I hold the celebrated philosopher who has 
taken the trouble to repeat my experiments. But though 
obliged to write in haste, I request you to accept of a few 
observations which the reading of his note has suggested to me. 
I find in Dr. Wollaston’s letter three poimts in which he has 
given an opinion different from that which I have advanced in 
my memoir. 
1. He affirms that the primitive form of'sulphate of iron is not 
a rhomboedron, but an oblique prism with a rhomboidal base. 
He founds his opinion on theoretical considerations. “ On 
examining,” says he, “ the modifications it assumes in its less. 
simple state, I have remarked a manifest difference in one 
direction of the erystal, proving that if the angular measures- 
were really equal, still the solid could not be considered as a 
rhomboid ; but must be viewed as a rhombic prism on account 
of some difference in its linear dimensions.” He then found by 
direct measurement that the angles are unequal. 
I eannot admit the first part of this statement. It appears to 
me that if the angles are equal, the solid is rigorously a rhomboid, 
whatever its extent may be in one direction or another. 
_ As to the second part, the difference of the angles which Dr. 
Wollaston announces, cannot, as he himself observes, in the 
least diminish the accuracy of my results, which he has in other 
respects found exact, and from which he draws the same conse- 
quences as I do. I may, therefore, admit the primitive form of 
sulphate of iron to be as he conceives it. But as the question is 
respecting one of the essential characters of this salt, I think it- 
better to enter into a short discussion respecting it. 
I have not time to verify the inequality of the angles with all: 
the care which this discussion demands. But such measures as 
{ have just taken with the common goniometer give me results 
sensibly equal ; and certainly I could not, with this instrument, 
commit an error of two degrees, as would result from the obser- 
vations of Dr. Wollaston. But I shall confine myself here to 
theoretic considerations, which are much more important than 
direct measures. In fact it is not by measures, the accuracy of 
of which depends upon the perfection of our instruments, that 
we are entitled to pronounce that the crystalline system of a 
body belongs to 2 rhomboid or a prism; but by the degree of 
symmetry which exists in all the faces, whether primary or 
secondary; which the crystals of the body presents. But in the 
sulphate of iron, we see all the modifications of which the crys- 
tals are susceptible, equally placed two and two, three and 
