368 Di- Brewster’s Reply to Mr Brooke's Observations 
In the preceding extract, it is taken for granted, that the prb 
mitive form of the sulphato-tri-carbonate of lead has been de- 
termined crystallographically to be the acute rhomboid , and I 
am said to have admitted this myself. I admit, it is true, that 
the crystals have to the eye the figure of acute rhomboids, pre- 
cisely in the same manner as the crystals of Sulphate of' Iron 
appeared to the practised eye of the celebrated Haiiy to be 
acute rhomboids ; but having found, notwithstanding this ap- 
pearance, that both the sulphato-tri-carbonate of lead , and the 
sulphate of iron, have two axes of double refraction, I conclude, 
and I persist in the truth of the conclusion, that both the sub 
phato-tri-carbonate of lead, and the sulphate of iron, have not 
the acute rhomb for their primitive form, but belong to the pris- 
matic system. Dr Wollaston detected the mistake of the Abbe 
Haiiy, and proved that sulphate of iron was a rhomboidal prism, 
and it is very likely that some other crystallographer may detect 
the mistake of Mr Brooke. But I deny that Mr Brooke has de- 
termined the primitive form of the sulphato-tri-carbonate to be an 
acute rhomboid. In his own original description of this mine- 
ral, in the Edin. Phil. Journal , vol. iii. p. 119* *, he says, “ The 
rhomboids are acute, measuring 72° 38' and 107° 80' ; and from 
not having found any other cleavage than one perpendicular to 
the axes of the crystal, I am induced to adopt this as the pri- 
mary form*. The natural planes of all except the most minute 
crystals , are more or less rounded, and consequently afford im- 
perfect reflexions.’’ 
In Mr Philips^ excellent work on Mineralogy, newly pub- 
lished, and enriched with the crystallographical observations 
of Mr Brooke, it is observed, that u the primary form of 
the sulphato-tri-carbonate of lead is considered to be an acute 
rhomboid.” The whole of Mr Brooke’s argument, therefore, is 
44 None of these forms,” I remark, 44 are capable of being derived from one ano- 
ther, and, therefore, each of them, as well as all their combinations, must remain 
entirely distinct from the rest.” His argument, such as it is, would have been 
gFeatly strengthened by quoting my own admission, and it would have been ad- 
vantageous to me, by shewing his readers that I was not ignorant of Professor 
Mohs’ System. 
• It appears from' this quotation, that one may with accuracy say, that a crys- 
tal is an acute rhomb, without maintaining that an acute rhomb is its primitive 
form, .. .< ... 
I 
