85 
TAB. CCXLIIT. 
ARGILLA _ hydrata. 
Hydrargilhte. 
Class 2. Earths. Order 1. Homogeneous. 
Gen. _— Argill. Spec. Hydrate of 
Div. 1. Crystallized. 
er 
We feel much pleasure in presenting our friends with a 
figure and account of the most perfect and rare specimen 
yet found of this substance, and particularly so, as its 
crystals are very neat and perfect on all sides, although they 
will yet try the patience of a Crystallographer to make them 
out, since they are so small, clear, brilhant, and dazzling 
to the sight, like Diamonds, that their facets become mul- 
tiplied. However, with a moderate lizht and good glasses 
they may be distinctly understood. At first sight they 
much resemble the octaédral crystal of Sulphur, with the 
apex of the pyramids truncated*. The Rev. Mr. Gregor 
was so good as to inform me that this mineral was raised 
* Although the largest crystals are not above half a line in length, yet 
‘my son was enabled to measure the angles by the help of a micrescope; and 
as he performed this by a new method, I mention it for the sake of such as 
might find such a mode useful. It is by placing the crystal in the forceps, 
or by any other convenient way, under the microscope, using one eye to 
examine the angles, while with the other they are compared with other 
large ones formed on paper, and placed several inches below; so that when 
a second line is drawn on paper, only differing a degree or two, it may 
readily be ascertained which is the most correct. The error of course can- 
not be much, and this is a perfection which I believe has not been before 
attained; nor could it have been exyj‘ected, in sucli minute crystals. 
