OXIDES 465 



well washed, and tlicu reduced into a fine pulp by rasping, grinding, or other suitable 

 means ; such pulp is then washed two or three times, by placing it in water, and well 

 stirring it therein, then permitting the pulp to subside, and running off the water. 

 The pulp thus obtained is next placed in an open vessel of lead, or wood lined with 

 lead, with as much water as will allow of the mixture being boiled freely, by means 

 of steam passed through leaden pipes placed therein. Into the mixture of pulp and 

 water, about 2 per cent, by weight (of the potatoes employed) of sulphuric acid 

 (oxalic acid acts more rapidly) is to be stirred in, which will be at the rate of from 

 8 to 1 per cent, of acid on the quantity of farina contained in the potatoes ; the 

 whole is now to be boiled for some hours, until the pulp of the potatoes is converted 

 into saccharine matter, the completion of this process being readily ascertained by 

 applying a drop of tincture of iodine to a small quantity of boiling liquor placed on 

 the surface of a piece of glass, when, if there be any farina remaining unconverted, 

 a purple colour will be produced. The saccharine product thus obtained is then 

 filtered through a horse-hair cloth, after which it is carefully evaporated in any 

 convenient vessel, until a gallon of it weighs about 14 or 14 J Ibs. ; it is now in a 

 proper condition to be employed in the manufacture of oxalic acid, by the application 

 of nitric acid, as in the case of operating from sugar or treacle. Horse-chestnuts, 

 deprived of their outer shells, are also applicable to the manufacture of oxalic acid 

 when treated in the way above described for potatoes. 



Instead of operating with sulphuric acid, the farina of potatoes and of chestnuts may 

 be treated with diastase, and converted into a liquor similar to that obtained after 

 evaporation from the farina and sulphuric acid before mentioned, using about the same 

 proportion of diastase as before directed for sulphuric acid. In this case the liquor is 

 made of the required strength at once, and the processes of filtration and evaporation 

 are rendered unnecessary. 



A new method of preparing oxalic acid, said to be cheaper than any other process, 

 has been introduced by Messrs. Eoberts, Dale, and Co. A mixed solution of 

 caustic soda and potash, in the proportion of two equivalents of the former to one 

 of the latter, is evaporated to a sp. gr. of about 1'35. It is then made into a thick 

 paste with sawdust, and heated, with constant stirring, on an iron plate, when the 

 mass iutumesces and disengages much inflammable gas. The mixture is then exposed 

 for some hours to a temperature of 400 F., and the grey powder thus obtained is 

 treated with water at about 60 F. The oxalate of soda which remains undissolved 

 'is then washed, and decomposed with slaked lime ; the oxalate of lime being 

 decomposed in turn by sulphuric acid, and the oxalic acid thus obtained in a free 

 state. 



Almost the only commercial article made from oxalic acid is the binoxalate of 

 potash or salt of sorrel. This substance results from the decomposition of carbonate 

 of potash by an excess of oxalic acid. The carbonate of potash is first dissolved in 

 hot water, and the oxalic acid added until the effervescence ceases ; after which a 

 similar quantity of oxalic acid to that previously employed is thrown in, and the solu- 

 tion is boiled for a few minutes ; and then it is set aside to crystallise. The crystals, 

 after being drained and dried, are fit for the market. 



Oxalic acid is employed chiefly for certain styles of discharge in calico-printing 

 (which see), and for whitening the leather of boot-tops. Oxalate of ammonia is an 

 excellent reagent for detecting lime and its salts in any solution. The acid itself, or 

 the binoxalate of potash, is often used for removing ink or iron-mould stains from linen. 



OX ; OXEN. A sub-tribe of animals belonging to the class Mammalia, order 

 Ruminantia, family Bovida, or hollow-horned ruminants. The ox appears from the 

 earliest periods to have been a domesticated animal. Its importance to man is great ; 

 not only does its flesh form an article of food, but its skin, its horns, and its hoofs are 

 employed in the arts and manufactures. 



OXFORD CHROME. An oxide of iron used in oil and water-colour painting. 



OXFORD CXiAV. An argillaceous or clayey deposit which is well developed in 

 the neighbourhood of Oxford. It forms the base of the Coral Eag or Coralline 

 Oolite, and extends across England in a north-easterly direction from Weymouth, in 

 Dorsetshire, to the river Humber. Its general character is that of a tough brown or 

 bluish-black clay, sometimes attaining a thickness of five or six hundred feet. It 

 furnishes admirable pasture ; a favourable example of which is afforded by the vale 

 of Blackmoor, in Dorsetshire, so famous for its dairy produce. H. W. B. 



OX GAIiXi. A mucous, transparent, ropy liquid, of a greenish brown colour, with 

 a bitter taste, obtained from the gall bladder of the ox. It has many uses in the arts. 

 See Watts's ' Dictionary of Chemistry ' (article BILE). 



OXIDE OF TIN". See TIN and PUTTY POWDER. 



OXIDES are compounds containing oxygen in definite proportions. 



They are usually divided into basic oxidfs. which unite with acids ; acid oxides, 



VOL. III. H It 



