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Mohs’s System of Crysiatlography 
25. Principal and accompanying series. — In different minerals, 
particularly in calcareous-spar, several rhomboids appear, which 
are not included in the preceding series, (20. 23.) They still, 
however, follow the same general law. We obtain them by 
multiplying the axis of any term in that series by |, |, From 
this process, three series of rhomboids arise ; of which that having 
I for its co-efficient is named ^e f rsU that having f the second, 
and that having | the third, accompanying series, in relation to 
the original, which is called principal series. The terms of 
the accompanying series are capable of being found by a de- 
duction similar to that used in the principal series ; but the nar- 
rowness of our limits will not allow us to exemplify this method. 
One instance of a single term in an accompanying series is s. 
Fig. 6. Plate xxiii. Haiiy. 
26. Isosceles six-sided pyramids. — Suppose Fig. 8. PI. VIII. 
to represent a crystalline form proceeding from iron-glance. If 
the faces P are enlarged till they comprehend the entire solid, 
the figure will be changed into a rhomboid. If the faces n are 
enlarged till they likewise completely include the whole solid, we 
obtain an isosceles six-sided pyramid. Fig. 9- PI. VIII. 
27. Propoi'tions to the rhomboid. — The faces of the pyramid 
uniting with those of the rhomboid, produce parallel edges of 
combination. The particular relation of the pyramid to the 
rhomboid, upon which this parallelism depends, is, that the ho- 
rizontal projections being equal, the axis of the pyramid is to that 
of the rhomboid as | to 1 . 
28. Series qf isosceles six-sided pyramids. — Whenever the 
axes of two such forms are found in the requisite proportions, 
those forms are always capable of producing parallel edges of 
combination : their absolute dimensions may be what’ they will, 
provided only they are both fitted to take a share in filling up 
the solid. Of an isosceles six-sided pyr'amid. which produces 
parallel edges of combination with a rhomboid in the manner 
just explained (26.), we say that it depends upon that rhomboid. 
Hence, upon any given rhomboid, there always depends an iso- 
sceles six-sided pyramid ; and upon any principal series of rhom- 
boids, a series of such pyramids, proceeding according to the 
law of the rhomboids, (20. — 22.) Any isosceles sixr-sided py- 
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