CHEOMIUM, OXIDE OP 801 



CHROMIC ACID. There are several methods of preparing this acid ; the sim- 

 plest consists in decomposing bichromate of potash by oil of vitriol : 1. An excess 

 of oil of vitriol is mixed with a warm solution of bichromate of potash ; the liquid is 

 poured off from the chromic acid, which separates in small red crystals ; the crystals 

 are drained in a funnel having its stem partly filled with coarsely-pounded glass, and 

 are afterwards dried on a porous tile under a bell glass : 2. Mr. Warrington mixes 

 10 measures of a cold saturated solution of bichromate of potash with from 12 to 15 

 measures of oil of vitriol free from lead, and presses the red acicular crystals which 

 separate as the liquid cools, between porous stones. If it be desired to remove the 

 last traces of sulphuric acid, the crystals should be redissolved in water, and a solution 

 of bichromate of baryta should be added in quantity just sufficient to throw down tho 

 whole of the sulphuric acid as sulphate of baryta ; the solution may be recrystallised 

 by evaporation in vacua : 3. Meissner prepares the acid direct from chromato of 

 baryta by digesting that salt with a quantity of dilute sulphuric acid, not sufficient 

 for complete saturation : the solution which contains chromic acid and acid chromate 

 of baryta is precipitated by the exact amount of sulphuric acid required, so that tho 

 solution is affected neither by sulphuric acid, nor by a salt of baryta : it is then 

 evaporated to dryness. 



Mr. Charles Watt obtains the acid by treating chromate of baryta with considerable 

 excess of pure strong nitric acid. The chromic acid is separated from the insoluble 

 nitrate of baryta by filtration through asbestos, and the solution is then evaporated 

 to dryness, while nitric acid is expelled. 



Chromic acid is obtained in quadrangular crystals, of a deep red colour ; it has a 

 very acrid and styptic taste. It reddens powerfully litmus-paper. It is deliquescent 

 in the air. When heated to redness, it emits oxygen and passes into the sesquioxide. 

 When a little of it is fused along with vitreous borax, the compound assumes an 

 emerald green colour. 



As chromic acid parts with its last dose of oxygen very easily, it is capable, in 

 certain styles of calico-printing, of becoming a valuable substitute for chlorine where 

 this more powerful substance would not, from peculiar circumstances, be admissible. 

 For this ingenious application, the arts are indebted to that truly scientific manu- 

 facturer, M. Daniel KxBchlin, of Miilhouse. He discovered that whenever chromate of 

 potash has its acid set free by its being mixed with tartaric or oxalic acid, or a 

 neutral vegetable substance (starch or sugar, for example), and a mineral acid, a very 

 lively action is produced, with disengagement of heat, and of several gases. The 

 result of this decomposition is the active reagent chromic acid, possessing valuable 

 properties to the printer. Water-solutions of chromate of potash and tartaric acid 

 being mixed, an effervescence is produced which has the power of destroying vegetable 

 colours. But this power lasts no longer than the effervescence. The mineral acids 

 react upon the chromate of potash only when vegetable colouring matter, gum, starch, 

 or a vegetable acid, are present to determine the disengagement of gas. During this 

 curious change, carbonic acid is evolved ; and when it takes place in a retort, there is 

 condensed in the receiver a colourless liquid, slightly acid, exhaling somewhat of tho 

 smell of vinegar, and containing a little empyreumatic oil. This liquid, heated with 

 the nitrates of mercury or silver, reduces these metals. On these principles M. Kcechlin 

 discharged indigo-blue by passing the cloth through a solution of chromate of potash, 

 and printing nitric acid thickened with gum upon certain spots. It is probable that 

 the employment of chromic acid would supersede the necessity of having recourse in 

 many cases to the more corrosive chlorine. H.M.N. 



Chromic acid is now used in microscopy for hardening the preparations of soft 

 tissues. 



CHROMXTE. See CHROME IRON-ORE. 



CHROMIUM. (Symbol, Cr ; Atomic weight, 26-27.) The metallic base of the 

 oxide of chromium. It may be obtained by exposing to a very high temperature, in 

 a crucible lined with charcoal, an intimate mixture of sesquioxide of chromium and 

 charcoal. The spongy mass obtained is powdered in an iron mortar, and mixed 

 with a little more sesquioxide of chromium (to oxidise as much as possible of the 

 carbon) ; it is then again exposed in a porcelain crucible to a very high temperature, 

 when a coherent metal is obtained. This metal is greyish in colour, hard and brittle, 

 and is magnetic at low temperatures. It has received no practical applications. 



CHROMIUM, OXIDE OF. There are several oxides of chromium, known to 

 chemists, but the only one used in the arts is the sesquioxido (Cr 2 3 ), known 

 commonly as green oxide of chromium. This has come so extensively into use as an 

 enamel colour for porcelain, that a fuller account of the best modes of manufacturing 

 it must prove acceptable to many of our readers. 



That oxide, in combination with water, called tho hydrate, may be economically 



Vol. I. 3 F 



