DIES FOB STAMPING 29 



DIASTASE. A white and tasteless nitrogenous substance, obtained by moisten- 

 ing pounded malt, and squeezing tho water through a bag. Albumen is precipitated 

 from tho turbid fluid by alcohol, and filtered. Then the diastase is precipitated by an 

 additional quantity of alcohol, and purified by re-solution and re-precipitation. 



Diastase has the property of acting like a ferment on starch, and converting it into 

 dextrine or into sugar, thus reducing the starch to a soluble form. One part of diastase 

 will convert 2,000 parts of starch into dextrine, and 1,000 parts into sugar. 



DXATHERMAN'OUS. Transparent to radiant heat, as rock-salt. 



DXCHROISCOPE. An instrument invented by Haidinger for examining the 

 dichroism, or pleochroism, of crystals. It consists of a rhombohedron of Iceland spar, 

 having a glass prism of 18 cemented to each end. This is enclosed in a brass tube, 

 one end of which carries a convex lens, whilst the other end is furnished with a screen 

 having a square aperture. On looking through this crystal two images of the aperture 

 are seen, and if a transparent dichroic crystal be held in front of the aperture the two 

 images appear of different tints. The dichroiscope may be used with advantage for 

 the discrimination of certain gems. If, for example, a stone be found to give two 

 differently-coloured images, we know at once that it cannot belong to the cubic system. 

 Thus, Oriental ruby may be distinguished from spinel-ruby, the former being doubly 

 refracting and dichroic, whilst the latter crystallises in the cubic system, and is 

 therefore not dichroic. See GEMS. 



DICHROISXVX. The property of exhibiting two colours. Many of the pheno- 

 mena belong to the conditions producing fluorescence. Some of the phenomena have 

 been referred to polarisation, but this requires examination. See FLUORESCENCE. 



DXCHROITE. A mineral otherwise known as lolite or Cordierite. It is called 

 Dichroite in allusion to its marked dichroism, or decided difference of colour observable 

 in its crystals when viewed in different directions. Chemically, it is a silicate of 

 alumina, magnesia, and protoxide of iron. 



DXD7IVXXUIVX. One of the rarer metals. It is found in connection with Lan- 

 thanum and Cerium. 



Solutions of salts of didymium give a characteristic absorption-spectrum containing 

 two black lines enclosing a very bright space. Mr. Greville Williams has detected 

 didymium in churchite by means of spectrum analysis, and has called attention to the 

 presence of this metal in some varieties of apatite. 



DIE-EARTH. A local term used in Coalbrookdale for the Wenlock Shale 

 because it lies below all the mining ground. 



DIES FOR STAMPING. (Coin*, Fr. ; Miinzstampeln, Ger.) The first circum- 

 stance that claims particular attention in the manufacture of dies is the selection of 

 the best kind of steel for the purpose, and this must in some measure be left to tho 

 experience of the die-forger, who, if well skilled in his art, will be able to form a tole- 

 rably correct judgment of the fitness of the metal for the purpose, by the manner in 

 which it works upon the anvil. It should be rather fine-grained than otherwise, and 

 above all things perfectly even and uniform in its texture, and free from spots and 

 patches finer or coarser than the general mass. But the very fine and uniform steel 

 with a silky fracture, which is so much esteemed for some of the purposes of cutlery, 

 is unfit for our present purpose, from the extreme facility with which it acquires great 

 hardness by pressure, and its liability to cracks and flaws. The very cross-grained, or 

 highly crystalline steel, is also equally objectionable ; it acquires fissures under the 

 die-press, and seldom admits of being equally and properly hardened. The object, 

 therefore, is to select a steel of a medium quality as to fineness of texture, not easily 

 acted upon by dilute sulphuric acid, and exhibiting an uniform texture when its surface 

 is washed over with a little aquafortis, by which its freedom from pins of iron, and 

 other irregularities of composition, is sufficiently indicated. 



The best kind of steel being thus selected, and properly forged at a high heat into 

 the rough die, it is softened by very careful annealing, and in that state, having been 

 smoothed externally, and brought to a table in the turning lathe, it is delivered to the 

 engraver. 



The process of annealing the die consists in heating it to a bright cherry red, 

 and suffering it to cool gradually, which is best effected by bedding it in a crucible 

 or iron pot of coarsely-powdered charcoal. In this operation it is sometimes sup- 

 posed that the die, or at least its superficial part, becomes super-carbonised or 

 highly-converted steel, as it is sometimes called; but experience does not justify 

 such an opinion, and probably the composition of the die is scarcely, certainly not 

 materially, affected by the process, for it does not remain long enough in the fire for 

 the purpose. 



The engraver usually commences his labours by working out the device with small 

 steel tools in intaglio ; he rarely begins in relief (though this is sometimes done) ; and 

 having ultimately completed his design, and satisfied himself of its general effect and 



