soda crystals until the arsenic is dissolved; this solution while still hot is poured into the copper solution, when a precipitate of copper arsenite will be obtained, and a little carbonate of copper also thrown down; sufficient acetic acid is now added to neutralise all the carbonate and leave a little in excess; the mixture is now allowed to stand for some time for the emerald-green to fully develop; in summer this may take from a week to ten days, in winter it will take about three or four weeks; when formed the green is filtered off, washed, and dried.

2. 8 lbs. of white arsenic is thoroughly mixed with water and then 8 lbs. of verdigris is stirred in; on standing for some time in a warm place emerald-green begins to form; when fully developed it is filtered, washed, and dried.

3. 50 lbs. of copper sulphate are dissolved in water, and to the solution is added 10 lbs. of lime dissolved in 20 gallons of vinegar; to the mixture is added 50 lbs. of white arsenic previously mixed into a paste with water; the mass is allowed to stand in a warm place until the emerald-green has formed, when it is finished as above.

4. Galloway's Process.-In the course of an article on emeraldgreen in the Journal of Science, a few years ago, Prof. Galloway described a process for the preparation of emerald-green on rather more scientific lines than either of the above processes, and which gives very good results. This process is carried out in the following manner :-A quantity (100 lbs.) of copper sulphate is dissolved in water, and sufficient sodium carbonate (282 lbs. of soda crystals or 12 lbs. of crystal carbonate) is added to precipitate one-fourth of the copper sulphate used in the form of copper carbonate; then acetic acid is added in sufficient quantity to dissolve this copper carbonate. There is thus obtained a solution containing copper acetate and copper sulphate in about the proportions 3 Cu SO4 + Cu 2 C, H, O2. The copper sulphate has now to be converted into copper arsenite; to do this the requisite amount of arsenic (60 lbs.) is dissolved by boiling in sodium carbonate (38 lbs. of crystal carbonate or 873 lbs. of soda crystals), which is rather less than is required to completely precipitate the copper sulphate in the first solution; the two solutions are heated to the boil and then the arsenic solution is run into the copper solution; the green is formed immediately and only requires filtering, washing, and drying, for use as a pigment. The quantities given above have been added by the author, and are not given in the original instructions. When carefully carried out this process gives excellent results.

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The fineness of the pigment can be regulated by altering the

strength of the solutions used; the weaker these are the finer is the precipitate and the more beautiful is the tint of the green produced. If during the precipitation of the green any tendency to form the yellow-green arsenite be noticed, the addition of the arsenic solution is stopped, and the mixture is boiled until all the yellow-green arsenite is converted into the blue-green emeraldgreen.

5. M. Camille Koechlin, in 1886, described in the Bulletin of the Industrial Society of Mulhouse, a process for the preparation of emerald-green. 100 grammes of copper sulphate are dissolved in 500 cc. of water; to this is added 1873 cc. of a solution of arsenite of soda; this solution contains 500 grammes of the salt in 1 litre of water. A precipitate of arsenite of copper is obtained, and is treated for one hour at from 104° to 122° F. with either 62 cc. of acetic acid of 11° to 12° Tw. or 31 cc. of pure formic acid; in either case a fine emerald-green is obtained. By using only half the quantity of formic acid a fine blue is obtained, a result which is not got with acetic acid.

6. Liebig's Process.-1 part of verdigris is dissolved by heat in acetic acid, then 1 part of arsenious acid, mixed with water, is added, and a yellow-green precipitate is obtained. The mixture is boiled for some time, and the green gradually forms; if necessary, a little acetic acid should be added from time to time to ensure that all the arsenite is converted into the aceto-arsenite; too great an excess of acid, however, should be avoided, as it would decrease the yield of emerald-green. As soon as the green is fully developed it is filtered off, washed, and dried. The drying of emerald-green must be done at as low a temperature as possible, as heat causes the tint to deteriorate.

Emerald-green is by no means a difficult colour to make; the first process described takes some time, but the last three are quick processes and give good results.

COMPOSITION AND PROPERTIES OF EMERALDGREEN.-Emerald-green is an aceto-arsenite of copper of somewhat variable composition, according to the process by which it has been made. The following analysis of a sample of English-made emerald-green will serve to show the average composition of this pigment:—

Leaving out of consideration the impurities, the formula for emerald-green deducible from the above analysis is-

7 Cu 2 C2 Hs O2, 3 Cu As2 04.

Emerald-green is a bluish-green of a very fine tint, quite different from any other known pigment, and very difficult to imitate; it is very opaque, and hence has good covering power; it works well in both oil and water, but best in the latter; kept in a dry place it is fairly permanent, and resists exposure to light and air, but in a damp place it turns brownish.

It is soluble in acids to blue solutions; in ammonia it also dissolves with the characteristic copper-ammonia colour; in solutions of caustic soda and potash it is also soluble; on boiling, a red precipitate of cupreous oxide falls down, a characteristic reaction of emerald-green.

Emerald-green cannot be mixed with pigments—such as cadmium-yellow, ultramarine, &c.-which contain sulphur, as this causes its discolouration, owing to the formation of black copper sulphide. With other pigments it can be mixed without any alteration. The use of emerald-green has been on the decrease of late years, partly owing to its poisonous character, due to its containing arsenic, although one authority states that there is no foundation for the statement that emerald-green is poisonous, and says that it has no poisonous properties whatever. The accounts of the poisonous action of emerald-green are very conflicting; some persons are much affected by emerald-green; even going into a room covered with paper printed with this pigment is sufficient to produce poisonous symptoms in them, while others are not affected at all; arsenic seems to be very peculiar in its toxic action, and much depends upon the physiological idiosyncrasies of the person.

ASSAY AND ANALYSIS OF EMERALD-GREEN.The colour or tint, body, and colouring power of emerald-green should be assayed for in the usual way.

To test for the purity or otherwise of a sample of emeraldgreen the following tests can be applied:-In hydrochloric acid it should completely dissolve with a yellow-green colour, and on diluting with water this colour should turn bluish. It dissolves in ammonia with a deep blue colour. In caustic soda it dissolves to a pale blue solution, from which, on boiling, a red precipitate of cupreous oxide falls down. The solution in hydrochloric acid should not give more than a faint precipitate with barium chloride, showing the absence of sulphates; through the solution a current of sulphuretted hydrogen should be passed for some

time and the precipitate of copper and arsenic sulphides obtained filtered off; the filtrate after boiling, to free it from excess of sulphuretted hydrogen, should give no further precipitate on the successive addition of ammonia, amonium sulphate, and ammonium oxalate, showing the absence of metals, such as iron, zinc, and calcium.

The presence of arsenic in emerald-green or other pigments is best detected by Marsh's test. This is carried out as follows:Provide a wide-mouthed bottle and fit it with a tight-fitting cork through which a piece of glass tube drawn out to a point is passed. Into the bottle, water, zinc, and sulphuric acid are placed. It is necessary that the two latter bodies be free from arsenic, as the ordinary commercial articles are very liable to contain arsenic which would interfere with the proper testing of any pigment for arsenic. By the action of the acid on the zinc hydrogen is evolved; this may be lighted as it issues from the glass jet, and will burn with a non-luminous flame. On pressing a piece of white porcelain down on the flame no brownish-black spot should be produced. The gas must not be lighted immediately it begins to issue from the jet, but a few minutes should be allowed to elapse before doing so, to allow the air in the bottle to be completely driven out; otherwise an explosion may ensue. If, after it has been proved that the gas flame produces no spot on a porcelain plate, the sample to be tested for arsenic be introduced into the bottle and the gas re-lighted, it will now be found to burn with a faintly luminous flame and will give a blackish-brown metallic looking spot on a piece of white porcelain pressed down on the flame; this stain is soluble in a solution of bleaching powder. Very small traces of arsenic can be detected by this test. Another test for arsenic is Reinsch's, which consists in heating the sample with hydrochloric acid and a clean copper plate; if arsenic is present the latter becomes covered with a grey deposit.

MINERAL GREEN.

Under this name and that of Mountain green is offered for use as a pigment the natural green mineral known as Malachite. There is also an artificial green pigment made under the name of mineral green.

Mineral green, Mountain green, or Malachite is a natural basic carbonate of copper found in many places-Cornwall, Siberia, Persia, Australia, &c. The ordinary commercial product comes from Siberia where it is found in the greatest abundance and in

large masses of a very fine colour. For use as a pigment the natural mineral is simply ground as finely as possible.

The mineral exists in two forms-in the one it forms compact masses of a fine yellowish-green tint; in the other form it is rather paler and more porous or powdery in character; the former is the most valuable as a pigment.

In composition it is a basic carbonate, containing on the average

Copper oxide, Cu O,
Carbonic acid, C O2,
Water, H2O,

and having the formula Cu CO2. Cu H2 02.

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It makes a good pigment, is fairly permanent, and works well both in oil and water. Its faults are those common to all copper pigments.

It dissolves in acids with effervescence and evolution of carbonic acid, and the solution gives all the characteristic tests for

copper.

Mineral green has been made artificially by several processes, of which the following are two examples :

(a) For preparing mineral green by this method the following materials are required :--2 cwts. of soda crystals, 1 cwts. of blue stone (copper sulphate), 70 lbs. of quicklime, 12 lbs. of white arsenic, and 4 ozs. of tartaric acid. Boil the arsenic and soda together until the former is dissolved; dissolve the copper in water; and slake the lime in water. Add the lime to the copper solution; then the arsenic and soda; and, finally, the tartaric acid. Keep the whole at a temperature of about 150° to 160° F. for some time until the colour is properly developed, then wash the pigment with clean water, filter and dry at a low temperature. (b) 14 ozs. of potash and 14 drms. of arsenic are dissolved in water; 1 lb. of sulphate of copper is dissolved separately in 2 gallons of water, and into this solution is poured the arsenic and potash solution; the green is precipitated and is collected and finished in the usual way.

No process of making mineral green artificially produces it of the same deep green tint as the natural variety; such prepared greens have a pale yellowish-green tint and are not so permanent as the natural variety. In their general features they resemble the verditer greens, but are rather brighter in tint and deeper in colour.

GREEN VERDITER.-This pigment is a basic carbonate