806 
Mr. William Phillips o?i the Modifications 
crystals appear at first sight to have no analogy.* Hence the va~ 
rieties may be expected to be almost endless. 
It is not the object of this paper to describe all the crystalline 
forms in which I have observed this substance. Its intention is 
much more limited ; being only that of noticing, and delineating 
the position, on the primitive form, of each of those secondary 
modifying planes, from the combinations of which have resulted all 
the numerous varieties which I have seen. I have attempted to 
measure the angles at which these secondary planes meet with those 
of the primitive crystal, and with each other, and have in most 
cases succeeded to a certain extent. 
According to Haiiy, the relation of the side B of the primitive 
crystal (PI. 16. Fig. 1), to the height G or H, is nearly that of 45 
* The sulphate of barytes is one among many substances, on the crystals of which it 
is not common to find the secondary planes equal in respect of size : this is more often 
observable in those, of which the primitive is a perfect geometrical solid, as the cube, 
the regular octohedron, &c. than on those which are not; as the varieties of paral- 
klopiped, &c. The secondary planes on the latter do not often present that perfect uni- 
formity, the observation of which on the crystals of certain substances doubtless deter- 
mined the Abbe Haviy to give each crystal, delineated on the plates to his “Traite &c.^, 
a separate designation. It cannot be denied that the names he has given, are often ex- 
pressive and sometimes of use, but I am tempted to believe that if that eminent mineralogist 
had extended his researches to the observation of a number of varieties of form, more 
nearly approaching their probable limit, the magnitude, and even upon the whole, the 
inutility of the labour would have become apparent. 
Some mode of distinguishing crystals from each other is essential; and as all those 
planes which form different angles with the faces of the primitive crystal, must be dif- 
ferent modifications of that crystal, each resulting from a separate and peculiar decrement, 
it seems reasonable that this decrement, or the planes which are its consequence, should 
form the distinctive character. This method, it is true, often places together crystals of 
a very different general form, arising from the various proportional dimensions of the 
secondary planes; it nevertheless soon becomes familiar to the observer, whom a little 
practice enables, if I may so express it to read off a crystal. It has the primitive planer 
together with the 1st, 4th, 7th, and 10th modifications, &c. 
