298 OnCiystallographrj. 



since it is as It were the common point round which the 

 three decrements have acted. 



In proportion as new lauiinas are afterwards applied to 

 the tormer, the points o, o are elevated, and point O is 

 lowered ; so that at a certain period we shall have the solid 

 repre?enlcd in fig. 3, in which the faces produced by the 

 decrement have become pentagons, such as o o i O e. 



Things being in this state, let us suppose a second de- 

 crement which concurs with the former, and which is pro- 

 duced by a simple range on the upper angle EA I, or 

 HA'K. The effect of this decrement will be to produce 

 two faces perpendicular to the axis ; and when it shall have 

 attained the point where these same faces shall intersect the 

 SIX faces parallel to the axis, which have the tirst decre- 

 3Tient for tlie generator, the secondary solid will be termi^ 

 naced, and will be presented under the form" of the regular 

 hexahedral prism (figs. 1 and 2) *. 



We have already said that ihis result was general, what- 

 ever was the measurement of the angles of the primitive 

 rhomboid. 



We now see wherefore, in the mechanical division of the 

 prism, the section pp oo (fig. 2) has its sides pp, oo pa- 

 rallel to each other, and at the same time to the horizon- 

 tal diagonal which goes from E to I (fig. 5), since the two 

 decrements taking place, one of them on the angle EOT, 

 the other on the^angle E A I, the laminae of superpositipii 

 should have their edges turned towards this same diagonal. 



In the case under consideration, and it is the most usual 

 one, the axis of the secondary crystal is longer than that of 

 the nucleus ; so that this nucleus having its lateral angles 

 contiguous to the panes of the prism, its summits are en- 

 gaged in the interior at a certain distance below the centres 

 ofihe bases. If we supposed that the two decrements had 

 the same epoch, then, the axis of the prism being equal to 

 that of the nucleus, the lateral angles and the sunnnits of 

 the latter would be tangent, the former to the panes, and 

 the other to the bases of the prism. Finally, if the decre- 

 ments on the upper angles of the nucleus had an epoch an- 

 terior to that of the other, which is the inverse of the first 

 case, the summits of the nucleus would still be contiguous 

 to the bases of the prism, while its lateral angles would be 



* We do not pretend here to detail the matinrr in which the crystal has 

 been formed, 1,'it solely the manner in which it is compoundt^. We shall 

 afterwards see how v/e may conceive that the process of crystallization is 

 combined with the order of the itructure. 



placed 



