430 ODOMETER 



OBSIDIAN. A glassy mineral ; so called, it is stated, from Obsidius, a Roman, 

 who brought it from Africa. It is a true volcanic glass, and occurs in streams, or in 

 detached masses near many volcanic mountains. It was largely employed by the ancient 

 Mexicans as a material for knives and other cutting-instruments. 



OCCZiUSIOW. A term applied by the late Professor Graham to the absorption 

 of gases by metals, and the shutting up of the same. Hydrogen in the nascent state 

 is absorbed in large quantities by palladium, and to some extent by other metals. 

 This gas is held by the metal until exposed to heat or other conditions, by which it 

 is liberated. Occlude is an old word in our language signifying to shut lip. See 

 Latham's ' Dictionary of the English Language.' 



OCHRE. ( Ocre, Fr. ; Ockcr, Ger.) Ochre is, truly, a peroxide of iron and 

 water ; but a native earthy mixture of silica and alumina, with oxide of iron in 

 various proportions, and sometimes calcareous matter and magnesia, is usually re- 

 garded as ochre. The term is applied, indeed, without any great degree of exactness, 

 to any combinations of the earths with iron, which can be employed for pigments 

 and the like. According as the colour varies, we have yellow, brown, and red ochres. 



In Cornwall considerable quantities of ochres are obtained by carefully washing the 

 ferruginous mud, which is separated from poor tin and copper ores after they have 

 been submitted to the action of the stamps and the ordinary processes of washing and 

 roasting. 



The iron paints formerly prepared by Mr. Wolston of Brixham must be regarded 

 as ochres. They are found in connection with iron lodes which exist in the rocks 

 around the coast. These paints have been employed for several years in the Royal 

 Naval Arsenals and other government establishments. The wood and iron huts of 

 our camps have been painted with them. They have also been employed for coating 

 the boilers of steam-engines. 



A large supply of ochre is obtained from the island of Anglesea, but the ochres of 

 Anglesea are not natural ; they are artificial productions, formed in the lakes, into 

 which scrap-iron is thrown to precipitate the copper contained in the water, and forms 

 very large accumulations of the oxide of iron, which is sold as ochre. There is now 

 a large demand for these iron-ochres, as they are extensively employed for the purifi- 

 cation of gas. 



In the more recent formations ochre occurs in beds some feet thick, which lie gene- 

 rally above the Oolite, are covered by sandstone and quartzose sands, more or less 

 ferruginous, and are accompanied by grey plastic clays, of a yellowish or reddish 

 colour. The ochry earths are prepared by grinding and washing ; in some cases they 

 are also exposed to the action of the fire, to increase the oxidation of the iron, and 

 deepen the colour. 



The following is a section of the ochre-pits at Shotover Hill, near Oxford, where 

 the Oxford ochre is obtained : 



Beds of highly ferruginous grit, forming the summit of the hill . 6 feet. 



Grey sand 3 



Ferruginous concretions 1 



Yellow sand 6 



Cream-coloured loam 4 



Ochre 6 inches. 



Beneath this there is a second bed of ochre, separated by a thin bed of clay. 



Reddle, employed for marking sheep in Devonshire, and a variety found near Rot- 

 terdam (which is much used for grinding spectacle-glasses at Sheffield), may be said 

 to belong to this class. See OXIDES OF IBON, for polishing. 



Bole, Armenian Bole, or Lemnian Earth, may be ranked with the ochres. See BOLE, 

 and TIEEHA DI SENNA. 



The Ochre of Bitry and Italian Souge are ochres which are found principally near 

 Vierzon and St. Amand (Nievre). The ochres from Holland are also much es- 

 teemed. 



It will thus be apparent that ochre occurs in all formations, from the earliest 

 known rocks where it is probably due to the decomposition of the sulphides of iron 

 up to the alluvial deposits of yesterday ; in many of which ochreous formations may 

 be watched in the progress. 



Ochre in mineralogy is a term applied to many products of decomposition, as, cobalt- 

 ochre, bismuth-ochre, chrome-ochre, antimony-ochre, &c. 



OCUBA V7AX. A vegetable wax, collected on the shores of the Amazon from 

 the fruit of the Myristica Ocuba. This wax is easily bleached ; and in Brazil it is 

 used extensively for candles. 



ODOMETER. An instrument which can be attached to the wheel of a carriage, 

 which, moving regularly with the wheel, indicates the distance passed over in any 

 journey. 



