OS 



'.& 



DIAMOND 19 



beyond the purpose of this Dictionary to enter into detailed descriptions of surveying 

 instruments, but the art of dialling will be explained.. The miner may be referred to 

 Budge's Practical Miners' Guide,' and to Eickard's ' Miners' Manual of Arithmetic 

 and Surveying.' See MINE SUEVETING. 



DXAXiXiAGE. Bronzite, Hypersthene, and Schiller-spar are often confounded under 

 this name. The name is .derived from 5ia\\ay^, difference, alluding to dissimilar 

 cleavage. It is thin, foliated, and easily cleavable ; laminse brittle ; colour, various 

 shades of green, grey, and brown, sometimes bronze and pearly metallic. Diallage is 

 now included by most mineralogists under the head of pyroxene. 



Of diallage-rock fine examples will be found near the Lizard Point, and beautiful 

 crystals of diallage are to be discovered in the Serpentine rocks near Cadgwith, in the 

 same locality. 



DXAXiXiOGXTE. Carbonate of manganese, a mineral usually occurring in rose- 

 red or pinkish rhombohedral crystals. See MANGANESE. 



DIALYSIS. The late Professor Graham applied this term to the separation of 

 certain substances by liquid diffusion. For example, if a mixture of gum and sugar 

 is placed in a dialyser, which is a piece of ' paper parchment ' stretched upon a hoop, 

 and this floated in water, three-fourths of the sugar will pass through the membrane in 

 twenty-four hours, without a trace of the gum. See Watts's ' Dictionary of Chemistry.' 



DXAIVXAGNETISM. As this term is now generally used in our scientific lan- 

 guage, it appears necessary to give a definition of it, although it is not our purpose to 

 enter on the consideration of any purely physical subject. 



The term was introduced by the late Dr. Faraday to express those bodies which did not 

 act as magnetic bodies do. If N and s (fig. 601) represent the poles of a horse-shoe mag- 

 net, any bar of a magnetic character, as iron, cobalt, or nickel, hung 

 up between them and free to move, will by virtue of the attracting 

 and repelling polar forces, place itself along the line joining the [| c 



two poles a b, which is called the magnetic axis. If, instead of a a 

 bar of iron, we suspend in the same manner a rod of glass, of NO c 

 bismuth, or of silver, it will arrange itself equatorially, or across 

 the line a b, as shown by the dotted lines, c d. All bodies in nature 

 appear to exist in one of those two conditions. The prefix dia 

 is used here in the same sense as in dia-meter. For a full explanation of all the dia- 

 magnetic phenomena, see De La Eive's ' Electricity,' and ' Eesearches on Diamagnetism 

 and Magne-crystallic Action,' by Dr. Tyndall. (Longmans, 1870.) 



DIAMOND. (Diamant, Fr. ; Diamant, Demant, Ger.) Experiment has deter- 

 mined that this beautiful gem is a peculiar (allotropic) condition of carbon. By burning 

 the diamond in oxygen-gas we produce carbonic acid ; and by enclosing the gem in a 

 mass of iron, and subjecting it to a strong heat, the metal is converted into steel, when 

 the diamond has disappeared. It has been shown that we can, by the agency of the 

 heat of the voltaic arc, convert the diamond into excellent coke, and into graphite ; but 

 although portions of coke are found to be sufficiently hard to cut glass, we have not 

 yet succeeded in making diamonds from coke. Sir Humphry Davy noticed that the 

 charcoal of one of the poles of Mr. Children's great voltaic battery was considerably 

 hardened, and he regarded this as an advance towards the production of that gem. 

 Eecently some experiments made by a French philosopher have advanced the dis- 

 covery another step : one of the poles of a voltaic battery being of charcoal and the other 

 of platinum, it was found that the fine charcoal escaping from the carbon pole and 

 depositing itself on the platinum pole was sufficiently hard to be used in the place of 

 diamond dust for polishing gems. The formation of the diamond in nature is one of 

 the problems which ' our philosophy ' has not yet enabled us to solve. Time is an 

 element which enters largely into Nature's works ; she occupies a thousand, or even 

 thousands of years to produce a result, while man in his experiments is confined to a 

 few days, or a few years at most. 



Although diamonds have been occasionally found in various parts of the globe, there 

 are only three places which can be strictly named as diamond districts, namely, a 

 portion of the Indian Peninsula, Brazil, and South Africa. India has been celebrated 

 from the most remote antiquity as the country of diamonds. Its principal mines are 

 in the kingdoms of Golconda and Visapur, extending from Cape Comorin to Bengal, 

 at the foot of a chain of mountains called the Orixa, which appear to belong to the 

 trap-rock formation. In all the Indian diamond-soils these gems are so dispersed 

 that they are rarely found directly, even in searching the richest spots, because they 

 are enveloped in an earthy crust, which must be removed before they can be seen. 

 The stony matter is therefore broken into pieces, and is then, as well as the looser 

 earth, washed in basins scooped out for the purpose. The gravel thus washed is 

 collected, spread out on a smooth piece of ground, and left to dry. The diamonds 

 are now recognised by their sparkling in the sun, and are picked out from the stone. 



c2 



