338 
SECOND REPORT- 1832 . 
such an occurrence is the history of a mineral which has been 
termed by Mr. Brooke the sulphato-tricarbonate of lead. It 
was placed both by Count Bournon and by Mr. Brooke among 
the rhombohedral or hexagonal forms, and therefore ought to 
have had but one optical axis. Sir David Brewster, however, 
found that it presented the phenomena belonging to two axes. 
The difficulty was solved when Mr. Haidinger subjected the 
substance to an exact crystallometrical examination. It then 
appeared that a figure which had been supposed to be a right 
hexagonal prism had not the exact dimensions which the sym¬ 
metry of that figure implies. Its sides, instead of making an 
angle of 90° with its ends, make an angle of 90° 29 '; and instead 
of making angles of 120° with each other, they make angles of 
120° 20'. The crystal had in fact precisely one of those forms 
from which its two optical axes would, by the rule, result*. 
Sir David Brewster had indeed already discovered a similar 
case in sulphate of potass ; which had been arranged as a rhom- 
bohedron by previous mineralogists : but when its optical exami¬ 
nation had indicated two axes, he found that the apparent 
bipyramidal dodecahedron of the rhombohedral system was 
composed of three prisms with angles of 114°. 
The memoir of 1818 of which we have spoken, contains also 
Sir David Brewster's very happy detection of the remarkable op¬ 
tical law on which the form of the curves seen in biaxal crystals 
depends; but on this and the other contents of this valuable me¬ 
moir I must, for the reason already referred to, forbear to dwell. 
The properties of doubly refracting crystals which affect 
all colours similarly, have now been reduced to a theory of 
singular beauty, which explains the most complex and apparent¬ 
ly anomalous parts of their details : but the properties of such 
crystals, which seem to select certain colours for their action, 
remain still to be traced to their most general laws. Here also, 
however, Sir David Brewster has done a large proportion of all 
that has been done. His memoirs on the absorption of Light by 
Crystals {Phil. Trans. 1816,) contain many curious facts on this 
subject. It is well known that certain tourmalines polarize the 
light which passes through them, to such an extent that they 
are commonly used as the easiest mode of obtaining polarized 
light. Agate and other substances were found, in like manner, 
* I have since found that this statement is not perfectly accurate; the solu¬ 
tion of the difficulty being somewhat different. There appear to have been 
included under the name sulphato-tricarbonate of lead, two different kinds of 
crystals belonging to different systems of crystallization. Some which Mr. Brooke 
found to be rhombohedral, Sir David Brewster found to have a single optical 
axis with no trace of composition; others were prismatic with two axes ; and 
thus Mr. Brooke’s original determinations were probably correct. 
