of the Primitive Crystal of the Sulphate of Barytes . 307 
to 46.* The accompanying drawing exhibits the primitive crystal 
according to those proportions, and the planes of each of the 18 
modifications on separate figures of less height, which was pre- 
ferred not only because they more nearly approach the proportions 
of the crystals, but also because the whole series is thereby exhibited 
at one view. I have added the representation of a crystal on which 
the planes of all the modifications are shewn in combination with 
those of the primitive crystal. This figure (20) is in part imagi- 
nary — that is to say, it does not represent any one that has fallen 
under my notice ; but I possess some on which are visible all the 
modifications delineated on the obtuse angle, except the 6th, and 
others shewing all those on the acute angle except the 13th and 
15th, and nearly in the proportions in which they are relatively 
drawn. All these crystals, however, are without the 17th and 
18th : the former is very common, the latter is very rare. 
I was induced to make a drawing of this, in part, supposititious 
crystal, from motives of utility. The crystalline forms of the sul- 
phate of barytes, though numerous, may readily be arranged, by 
the assistance even of the common goniometer, in a series com- 
mencing with the primitive crystal, and terminating with those 
crystals which exhibit the greatest number of planes. In forming 
such an arrangement, I believe that a reference to this figure will 
be found of some use, especially if also some attention be given to 
the annexed tabular view of all the measurements I have been able 
to obtain. 
* I have heretofore shewn (Gcol. Trans, vol. iv. p. *233.) that the measurements of the 
angles of the primitive crystal by the reflecting goniometer, differ from those given by the 
Abbe Ilaiiy : hence it is probable that those of the secondary planes, and consequently 
the height of the prism, which lie has deduced from calculation founded upon what ap- 
pears to bo an inaccurate measurement, are inaccurate also, 
2q2 
