XIII. On the Modifications of the primitive Crystal of the Sulphate 
of Barytes . 
By WILLIAM PHILLIPS, Esq. f. l. s. 
MEMBER OF THE GEOLOGICAL SOCIETY. 
£Read November 6th, 1818.] 
It is observed by the Abbe Haiiy in his <e Tableau Comparatif 
&c/’ that the crystalline forms of this substance, of which only 
thirteen varieties had been noticed when his “ Traite &c.” was 
published, amount to the number of sixty- three, in consequence of 
the researches of M. Mabru among the crystals abounding in the 
departments of the Puy de Dome and Cantal. In my collection 
there are about 120 varieties of form, of which the greater part are 
from the north of England. 
The primitive crystal, which is a right quadrangular prism with 
rhomboidal terminations, is subject to several modifications ; the 
varieties delineated by the Abbe Haiiy result from various combi- 
nations of the planes of eight modifications, with those of the 
primitive prism ; whilst those in my possession result from eigh- 
teen. It is impossible even to imagine how many varieties of 
form may arise from so considerable a number of secondary or 
modifying planes, when it is considered that the enlargement of 
the planes of any one modification, may and does often produce in 
a crystal, so great a difference in general form, from another pre- 
senting the same planes, but of different relative size, that the two 
Vol. V. 2 Q 
