D I A 



[ 132] 



D I A 



placed in a perpendicular position, 

 has its spriral volutions in an oppo- 

 site direction to the motion of the 

 index of a clock, and resembles 

 what is called a sinistral, or left- 

 handed screw. The sinistral shells 

 are sometimes termed heteroclitical, 

 and heterostrophe shells. There 

 has been considerable confusion 

 amongst conchological writers in 

 describing the position in which 

 shells should be held, to ascertain 

 the right from the left sidfc, &c. 

 Perhaps, the most simple plan is, 

 to place the apex of any spiral 

 shell towards the eye with the 

 mouth downwards ; dextral shells 

 will then be found to have their 

 aperture on the right side of the 

 axis', sinistral shells, on the con- 

 trary, will have theirs on the left 

 of the axit. 



DI'ABASE. A crystalline granular, 

 sometimes porphyritic, or even 

 a slaty, mixture of augite and 

 labradorite or oligoclase, mostly 

 with some chlorite. Jukes. 



DIADE'LPHIA. (from /? and a 



Gr.) The seventeeth class of 

 plants in the artificial system of 

 Linnaeus. The stamens are united 

 into two parcels at the base. This 

 class has papilionaceous flowers 

 and leguminous fruits. Familiar 

 specimens will be found in the 

 garden pea, bean, &c. &c. 



DIADF/LPHOTJS. Having its stamens 

 united into two parcels at the 

 base ; belonging to the class Dia- 

 delphia. 



DTA'GONAL. (StaryoWos, Gr. db angulo 

 ad angulum perductm : diagonius, 

 Lat. diagonal, Fr. diagondle, It.) 

 A line reaching from one angle to 

 another, so as to divide a parallello- 

 gram into equal parts. Diagonals 

 principally belong to quadrilateral 

 figures. 



DIA'GONALLY. (diagonalement, Fr. 

 diagonalemente, It.) In a diagonal 

 direction. 



Di' ALL AGE. Schiller spar ; a variety 

 of augite or crystallized serpentine. 

 The colour of diallage is dark- 

 green. 



DI'AMOND. (diamant, Fr. diamante, 

 It. aSapa*}, Gr. adamas, Lat.) The 

 hardest and most valuable of all 

 the precious stones. Some min- 

 eralogists form a family of the 

 various sorts of diamond, under 

 the title Diamond Family, placing 

 it in the class of earthy minerals : 

 others place the diamond amongst 



~ those minerals termed combustible, 

 of which the basis is either sulphur 

 or carbon. Strange as it may 

 appear, diamond consists of pure 

 carbon. If the best charcoal be 

 burnt in oxygen, carbonic acid gas 

 is formed, the weight of which is 

 nearly equal to that of the charcoal 

 and the oxygen, there being a 

 small residuum of earthy ashes 

 left after the combustion ; but if, 

 in like manner, a diamond be 

 burnt in oxygen, carbonic acid gas 

 is equally the result, though, in 

 the latter case, there is no residuum, 

 and the carbonic acid gas obtained 

 is precisely equal in weight to the 

 two elements, the oxygen and the 

 diamond. Why, or how, it is that 

 the same elementary substance can, 

 with little or no addition, form 

 two such excessively dissimilar 

 bodies as diamond and charcoal, 

 the former the hardest and clearest 

 body in nature, the latter a mere 

 black soft, brittle mass, is a 

 mystery beyond our finite powers 

 to comprehend. The primitive 

 crystal of the diamond is the 

 regular octahedron, each triangular 

 facet of which is sometimes re- 

 placed by six secondary triangles 

 bounded by curved lines ; so that 

 the crystal becomes spheroidal, 

 with 48 facets. When rubbed, the 

 diamond shews positive electricity. 

 It reflects all the light falling on 

 its posterior surface at an angle of 

 incidence greater than 24 13', 



