404- Dr. T. Thomson on the Minerals 



atom of chloride of lead and eight atoms subsesquiphosphate 

 of lead. There is usually a small quantity of oxide of iron 

 present, to which in all probability the colour of the mineral 

 is owing. I examined five specimens of this mineral from 

 Lead Hills; the average quantity of chloride of lead in them 

 was 10^ per cent. ; the average quantity of protoxide of iron 

 was 1^ per cent. ; the rest was a compound of 1^ atom of 

 oxide of lead and one atom of phosphoric acid. 



The colour of these specimens was grass-green, olive-green, 

 yellow and brown. The specific gravity varied from 6'70016 

 to 6*5781. None of them contained any phosphate of lime. 

 This was the case also with the specimens analysed by Wb'hler 

 and Berthier, and with those that Klaproth had analysed. 

 But some years ago Mr. Charles Kersten of Freyberg pub- 

 lished an analysis of seven specimens of phosphate of lead 

 from various localities, five of which contained phosphate of 

 lime, varying in the different specimens from 12 per cent, to 

 0'59 per cent. The specific gravity of these specimens, which 

 contain much phosphate of lime, is lower than of the others. 

 That which contains 12 per cent, has a specific gravity of 

 only 5-092. 



Some months ago I got two specimens of phosphate of lead 

 from Mr. Brown, locality Lead Hills, still lighter than any 

 I had before seen. The first specimen was of a light greenish- 

 yellow colour, and a slaty texture, and had a specific gravity 

 of only 5'366. It contained no less than 15 per cent, of 

 phosphate of lime. 



The other specimen was dark-green, and constituted a bo- 

 tryoidal concretion upon the surface of the first kind. Its 

 specific gravity was 5 '970, and it contained rather more than 

 nine per cent, of phosphate of lime. 



From the great difference of the quantity of phosphate of 

 lime in different specimens, it would seem rather to be me- 

 chanically mixed with than chemically combined with the 

 phosphate of lead. 



7. Cupreo-sulphato- carbonate of lead. This is the scarcest 

 of all the ores of lead which occur at Lead Hills. It has 

 a fine green colour, precisely like that of malachite, and 

 occurs most frequently in mineral crystals, having for their 

 primary form, according to Brooke, a right rhombic prism 

 with angles of 95 and 85. It was first noticed by Mr. 

 Sowerby under the name of green carbonate of copper. It 

 was first described and analysed by Mr. Brooke in 1820. 



I could only employ for analysis 3'29 grains of this mineral ; 

 but as the analysis is simple, and I employed every precaution 

 I could think of to avoid error, I have reason to believe that 



