160 Mr Brooke's Remark s on the Sulphate -tri-carbonate of' Lead, 
frequently small crystals of carbonate of lead , accompanying 
the sulphato-tri-carbonate, which present some striking analogies 
with Mr Haidinger’s figures 
A twin-crystal of carbonate of lead now lies before me, of 
which the plane of junction corresponds very nearly in position 
with that of Mr Haidinger's fig. 1 ., and where it passes through 
the lateral planes, an obtuse re-entering angle is produced. If 
the reader will refer to Mr Phillips's measurements of carbonate 
of lead in his Mineralogy, p. 339, he will observe, that there are 
two planes i and m, fig. 3, truncating the edge of h, and that 
Mr Phillips gives for the measurement of these planes on h 
h on i 151° 21' 
h m 121 26 
If we turn to Mr Haidinger's measurements, we shall find he 
gives 
a on l 151° 10' 
a m 132 15 
a n 121 12 
Although I have repeatedly found a plane corresponding to his 
m, it is remarkable that it has never been accompanied by l or 
n. Still, however, these may exist on Mr Haidinger's crystal, 
and their near agreement in measure with Mr Phillips's i and m, 
may be merely accidental. I shall only add, that Mr Haid- 
inger's oblique planes cl are by no means incompatible with the 
secondary form of a rhomboid, when it is recollected that one 
of two symmetrical planes will frequently appear singly on a 
crystal. The plane d might belong to the modification I have 
denoted byj^ an example of which occurs in Haiiy's fig. 139- 
of carbonate of lime, in the late edition of his Mineralogy. 
If the primary form, therefore, of the mineral in question be 
not, as the figures of the crystals indicate, a rhomboid, some 
other substitution must be made than that which Mr Haidinger 
has proposed. 
1 
