REPORT ON CHEMISTRY. 
429 
In the arseniate of ammonia, M 7 or M ;/ = 85° 54 / , while in 
the phosphate it is only 84 9 off. 
Now, according to Mitscherlich, the errors of observation in 
good crystals, and the difference between the angles of dif¬ 
ferent crystals of the same mineral in good specimens, rarely 
exceed Iff , though they sometimes amount to 20'. In the same 
crystal differences equally great sometimes occur. In a crystal 
of binarseniate of potash, he found a difference between the 
inclinations of two series of similar faces, of 59 '. These differ¬ 
ences depend on the nature of the surfaces, and the degree of 
concentration of the mother liquor in which they are formed. 
Plesiomorphism .—As the differences between the angles of 
the carbonates and sulphates above quoted cannot be accounted 
for by any accidental causes similar to those now detailed, some 
crystallographers have been led to reject the term isomorphoas 
as applied to such crystallized compounds, and to substitute 
in its place the term plesiomorphous. 
To this term there can be no objection while it is employed 
only to denote certain groups of crystals belonging to the sy¬ 
stem, whose forms approximate without becoming identical, but 
it can never supersede the term isomorphous, or be employed 
to designate groups of elementary substances, or of their bi¬ 
nary compounds. Because lead in the plumbo-calcite * re¬ 
places lime without altering the form of the crystal, we infer 
that these two bases, lead and lime, are isomorphous. But, 
because we find that the angle of arragonite differs 1° 8' from 
that of carbonate of lead, were we to infer that the bases them¬ 
selves are only plesiomorphous, we should have the same sub¬ 
stances at once plesio- and iso-morphous, which is a contra¬ 
diction. While then the cases in which the angles of crystals 
of similar compounds of the members of an isomorphous group 
differ from each other, are so few in number as to constitute 
only exceptions to a general law, the cause of the anomaly must 
be sought for in something else than a difference in the forms 
of the ultimate atoms. The fact itself is a very interesting one, 
—that there exist plesiomorphous groups of similar compounds 
of isomorphous bodies. 
Homoiomorphis7n,-—lL\\e differences under discussion have 
given rise in Germany to another term, homoiomorphous (bjxoiog 
like, iu,op<pY] shape), of more extended signification than plesio¬ 
morphous. It groups together crystalline forms differing 
widely in their angles t, provided they belong to the same 
* For an account of this interesting mineral, see Brewster’s Ed'mb. Journ. of 
Science, N. S. No. XI. p. 79. 
f See a paper by Kobell, in Schweigger’s Jahrbuch, iv. p. 410, where several 
uch groups are to be found. 
