662 Scientific Proceedings, Royal Dublin Society. 
To compare with the above we have the very remarkable 
resemblances obtaining between a new addition-compound of 
camphorie acid with acetone and camphoric anhydride, which have 
been traced by Pope.’ Both erystallize in the ortho-rhombic system 
and display the same axial values in two directions, while they 
differ as to the third: the cleavages too, the planes of which belong 
to the zone throughout which the angular values are the same in 
both, correspond very closely, and so do the optical characters and 
the face-markings on the crystals. 
The kind of relation just referred to is very suggestive in 
connection with homologous series of carbon compounds, it being 
possible to form series of groupings of balls, each term of which is 
derived from the previous term by adding balls symmetrically at 
the end so as to elongate the group without altering its general 
character. The fitting of similar groups together side by side, as 
well as many features of the groups taken individually, will be 
found to furnish great similarities if the different types forming 
such a series are taken successively, and the geometrical features 
compared. 
If only one, or a few of the equivalent atoms in a compound 
are replaced by others, the crystalline system generally changes, 
and usually in such a way that the new system possesses less 
symmetry than the old. The influence of substitution on the 
crystalline form of benzene and its derivatives has been the most 
completely investigated. 
In the case of substitution of hydroxl- or nitro- groups for 
hydrogen atoms in the benzene molecule the length of only one 
axis changes considerably, those of the two others remaining 
practically constant.’ 
In connection with the consideration of the effects of partial 
similarity of composition and grouping on the general symmetry 
of an assemblage, it is very important to observe that groups are 
conceivable in which a high symmetry would be presented were it 
not for some slight defect in form or arrangement, and it is evident 
that, in the case of such groups, closest-packing may be attained by the 
1 Transactions Chemical Soc., 1896, p. 1696. 
2 Pope’s translation of Fock’s “‘Chemische Krystallographie,’’ p.176. Comp. Groth’s 
Zeitschr. f. Kyrst., 8. p. 284. Comp. also ante, p. 619. 
es 
