TllAXSACTIOKS OF SECTION C. 875 



regarded as an assemblage of sucli forms, the author, continuing the inquiry on 

 which he read a paper at the Toronto meeting, considers the ratio between the 

 space occupied by sucli an assemblage and the volume of the interspaces between 

 the atoms. By a comparison with the atomic volume he obtains thus a series of 

 numbers which he regards as indicating the relative dimensions of the atoms of 

 the following elements : C, Si, Pb, Th, Sn, P, As, Sb, Bi, S, I, Fe, Ni, Pd, Pt. 



9. Leadhillite in Ancient Lead Slags from the Mendip Hills. 

 By L. J. Spencer, M.A., F.G.S. 



Lead ores have been worked in the Mendip Hills (East Somerset) ever since 

 the time of the Romans ; but during the present century operations have been 

 chiefly confined to the reworking of the old waste heaps of slags and slimes. 

 From these heaps upwards of 9,000 tons of lead were extracted during the ten 

 years ending 1880. The material now being worked at Priddy has the appear- 

 ance of a brown earth : it contains fragments of charcoal and limestone, and about 

 6 per cent, of lead as carbonate. Embedded here and there in this material are 

 blocks consisting of devitrified slag, partially fused galena, and fragments of 

 charcoal ; and in the cavities numerous small crystals of cerussite (PbCOj) and 

 anglesite (PbSOJ and, less frequently, of leadhillite. 



Leadhillite has not been before observed under such conditions. In the 

 Roman lead slags at Laurion, in Greece, which have been in contact with sea- 

 water, Lacroix has noted the following secondary minerals : matlockite, penfieldite, 

 laurionite, fiedlerite, phosgenite, cerussite, hydrocerussite, and anglesite. 



The colourless crystals of Mendip leadhillite have, perpendicular to the perfect 

 basal cleavage, an acute negative bisectrix with an optic axial angle in air of 

 2E = 72|°; at a temperature of 97° C, 2E = 70|°. The frequent twinning and 

 the goniometric measurements (which are, however, not very good) are not incon- 

 sistent with the orthorhombic symmetry insisted upon by Miller. The basal 

 planes of complicated twin crystals always give a single sharp reflected image, 

 which is not the case with twin crystals of ordinary monosymmetric leadhillite. 

 A few crystals are optically imiaxial. 



There therefore seem to be three kinds of leadhillite, all of which are identical 

 in outward appearance : («) Monosymmetric, with the optic axial angle 2E = 20° ; 

 (6) rhombohedral (?) and optically uniaxial (susannite) : (c) orthorhombic, with 

 2E = 72J°. 



Before 1874 the formula for leadhillite was given as PbS04.3PbC03, and that 

 now usually accepted is PbSO4.2PbCO3.Pb(0H), ; but no two of the several 

 analyses that have been made are in close agreement, and other formulae have 

 been proposed. Doubtless each of the above kinds has a definite chemical com- 

 position, and the variations shown by the different analyses are possibly due to 

 the fact that two {{a) and {b) or (/>) and (c)) of the three kinds may occur together 

 in the same crystal, as observed by Bertrand and by myself in specimens from 

 Leadhills. It will therefore be necessary to examine optically each fragment that 

 is collected for future analyses of leadhillite. 



10. Suj>pleinentary List of British Minerals. 

 By L. J. Spencer, M.A., F.G.S. 



During the forty years which have elapsed since the publication of Greg and 

 Lettsom's ' Manual of the Mineralogy of Great Britain and Ireland ' a considerable 

 number of species, variety, and other names have been added to the list of minerals 

 occurring in the British Isles. In 1858 Greg and Lettsom recognised 241 British 

 species ; but of these only 209 are given as numbered species by Dana in the 

 sixth edition (1892) of his ' System of Mineralogy.' To this list may now be 

 added 84 more, bringing the total number of British species up to 293, as com- 

 pared with the total of 824 known mineral species recognised by Dana in 1892, 



