241 
Philosophical Transactions,’ a theory of the formation of the 
ordinary tetrahedron and octahedron, by means of spheres 
having a simple mutual attraction for each other; their arrange- 
ment may be best described as that of cannon balls piled into 
an equilateral triangular pyramid. 
‘¢ T shall now endeavour to show that this theory is faulty, 
inasmuch as the molecules, if under the influence of mutual 
attraction, would never assume such an arrangement; for if 
any four molecules become attached they will form a triangular 
pyramid, and if a fifth become attached it will touch three 
others ; but in his arrangement the fifth molecule touches only 
two others, and is in the same plane with three of them, a po- 
sition which is certainly not that of equilibrium. 
‘«‘ Haity considered the ultimate atom to have the same 
_ form as the solid obtained by cleavage, he did not explain 
under what law of attraction these particles become aggrega- 
_ ted. His theory totally failed in accounting for the formation 
of crystals which have a tetrahedral cleavage, inasmuch as 
_ tetrahedrons will not fill space. 
‘<¢ Dana considers the molecules, in the first system, to be 
spherical and possessed of six poles or centres of force, situated 
at the extremities of three rectangular axes ; in the other sys- 
_ tems these sphere, become spheroids or ellipsoids. By means 
of this bi tothedtaiie satisfactorily explained the cubical and 
prismatic formations, and pointed out, like Haiiy, how the 
secondary forms will result from certain decrements at the 
edges or angles. 
‘¢ He also accounted for the occurrence of twin crystals by 
4 " supposing that two molecules may unite at a point of equili~ 
_ brium between two or three poles. He did not, however, show 
_ why decrements take place, nor is it possible on his hypothesis 
: to explain how differerit ‘cleavages can exist in the same sys- 
pA tem. His ideas are directly borrowed from Sir D. Brewster, 
_ who thus expresses himself :—‘ The phenomena of cleavage, 
_ and of hemitrope forms, would clearly indicate that the inte- 
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VOL. VI. Y 
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