DIOPTASE 33 



and acted upon by a weight which may be made to bear upon any point of its gra- 

 duated arm. 



Chevreul made a useful application of the digester to vegetable analysis. His 

 instrument consists of a strong copper cylinder, into which enters a tight cylinder of 

 silver, having its edge turned over at right angles to the axis of the cylinder, so as to 

 form the rim of the digester. A segment of a copper sphere, also lined with silver, 

 stops the aperture of the silver cylinder, being applied closely to its rim. It has a 

 conical valve pressed with a spiral spring, of any desired force, estimated by a steel- 

 yard. This spring is enclosed within a brass box, perforated with four holes, which 

 may be screwed into a tapped orifice in the top of the digester. A tube screwed into 

 another hole serves to conduct away the condensible vapours at pleasure into a 

 Woulfe's apparatus. 



DIKA-BREAD. A food made from the fruit of the Mangifera Gabonensis, a 

 tree indigenous to the country from Sierra Leone to the Gaboon. It is one of the 

 Anacardiacea. White almond kernels are found in the fruit ; these are bruised and 

 pressed to form the Dika-bread. 



DIKE. See DYKE. 



DILATATION. The increase of size produced in bodies by the agency of heat. 

 See EXPANSION. 



DILI.. The Anethum graveolens, an umbelliferous plant, containing an essential 

 oil. 



DILLltflTE. A hydrous silicate of alumina, occurring with diaspore at Dilln, 

 near Schemnitz, in Hungary. 



DILUVIUM. Those accumulations of gravel and loose materials, which, by some 

 geologists, are said to have been produced by the action of a diluvian wave or deluge 

 sweeping over the surface of the earth. I/yell. 



The term was formerly applied to the superficial deposits, now. commonly called 

 c drift,' and was used to distinguish them from the still more recent accumulations 

 known as alluvium. 



DIMITY is a kind of cotton cloth originally imported from India, and now manu- 

 factured in great quantities in various parts of Britain, especially in Lancashire. Dr. 

 Johnson calls it dimmity, and describes it as a kind of fustian. The distinction between 

 fustian and dimity seems to be, that the former designates a common tweeled cotton 

 cloth of a stout fabric, which receives no ornament in the loom, but is most frequently 

 dyed after being woven, Dimity is also a stout cotton cloth, but not usually of so 

 thick a texture ; and is ornamented in the loom, either with raised stripes or fancy 

 figures : it is seldom dyed, but usually employed white, as for bed and bed-room fur- 

 niture. The striped dimities are the most common; they require less labour in 

 weaving than the others ; and the mounting of the loom being more simple, and con- 

 sequently less expensive, they can be sold at much lower rates. 



DIMORPHISM;, (dis, two ; morphe, form.) ,This name is applied to bodies which 

 will crystallise in two forms. Sulphur, for example, will usually crystallise in the 

 rhombic system ; but will when melted under certain conditions form monoclinohedric 

 crystals. Carbon appears in nature as the diamond and graphite, and carbonate of 

 lime as calc-spar and aragonite. Many chemical compounds of the same compositions 

 are susceptible of crystallisation in two essentially distinct forms. 



In Biology, the tendency of some organisms to exhibit two distinct forms in their 

 essential organs is called dimorphism. Thus Mr. Darwin has shown that some species 

 of Primula are dimorphic, one form having a long style and short stamens, and the 

 other form having a short style and long stamens. A check is thus given to self- 

 fertilisation ; perfect fertilisation occurring only when tlje two dimorphic forms are 

 crossed, the pollen of the short-stamened variety being applied to the stigma of the 

 short-styled form, and conversely the pollen of the long-stamened variety to the 

 stigma of the long-styled form. 



DIMORPHITE. A sulphide of arsenic, containing As 2 S 3 (As'S 3 ). It occurs at 

 the Solfatara, near Naples. 



DIM" AS BRICKS. Fire-bricks made of Dinas sand, and much used in the con- 

 struction of the copper-smelting furnaces of South Wales. The rock known as ' Dinas 

 sand ' is found in the Vale of Neath in Glamorganshire, and consists almost wholly of 

 silica. Having been ground, the material is mixed with water, with addition of about 

 one per cent, of lime, and is pressed into the shape of bricks, which are then fired. 

 The granules of quartz are cemented together by a slightly-fusible compound formed 

 by the action of the lime. 



DIOPSIDE. A name applied to a clear green crystalline variety of pyroxene or 

 augite. It has occasionally been cut as a gem-stone, but is too soft to be of much 

 value. 



DIOPTASE, or Emerald Copper, occurs in fine emerald green transparent crys- 



VOL. II. D 



