antimony and lead, and can be prepared of various shades and from different materials.

(a) 1 part of tartar emetic, 2 parts of lead nitrate, and 4 parts of salt are intimately mixed together, and the mixture placed in a crucible and heated to fusion, at which point it is kept for 2 hours; after which the fused mass is treated with water to wash out the soluble alkaline salts present in it, and the pigment is dried at a gentle heat.

(b) 1 part of tartar emetic, 2 parts of red lead, and 4 parts of salt are treated as above.

(c) 3 parts of antimony, 1 part of zinc oxide, and 2 parts of red lead are heated to fusion in a covered crucible for 4 hours; after which they are ground under water and the pigment dried at a gentle heat.

(d) A process of preparing an antimony-lead yellow from the dross of lead refining was patented in 1858, by Dick, which consisted in mixing this dross (which is a mixture of the oxides of lead and antimony with some small quantities of other impurities) with salt, fusing the mass for 2 to 3 hours, then washing it well with water and drying the pigment.

(e) A yellow not unlike Naples yellow has been made from the three oxides of tin, lead, and antimony, by calcining for 3 to 4 hours in a crucible a mixture of 2 parts of levigated crude antimony, 2 parts of tin ashes, and 5 parts of white lead; or 1 part each of tin ashes, litharge, and antimony may be used.

(f) 1 part of type-metal, 2 parts of potassium nitrate, and 4 parts of salt are fused together, and the fused mass treated as in process a.

(g) Processes for preparing antimony yellows were patented by Hallet and Stenhouse, in 1861, as follows:-1. Antimony ore was calcined and then mixed with oxide of zinc and litharge, the mixture being fused. 2. A mixture of type-metal and zinc oxide are fused together.

The yellows made by the above methods have been sold under various names-Naples yellow, Jaune, solid yellow, antimony yellow, &c. They were rather favourite colours at one time with artists, but their use has become nearly obsolete. They are bright colours, although not equal to the chromes in this respect; are fairly fast to light, but, like all lead colours, are affected by sulphureous gases; iron has an injurious effect upon these colours so that they cannot be ground in iron mills with safety. They are equally useful as oil- or water-colours, and are of good body or covering power.

KING'S YELLOW.

This yellow, which at one time was in extensive use, is the trisulphide of arsenic, As, S. It is found native as the mineral orpiment, which is sometimes ground up and used us a pigment. The artificial colour is usually made by precipitation, but it can also be made by sublimation.

1. By Precipitation.-(a) Arsenic is dissolved in hydrochloric acid and a current of sulphuretted hydrogen gas passed through the solution; a fine yellow precipitate of the colour is obtained, which is collected and dried at a gentle heat. (b) A fine yellow pigment, formerly sold under the name of Royal yellow, is made by mixing 2 parts of barium sulphate with 1 part of charcoal and calcining the mixture, when barium sulphide is formed; this is ground with orpiment and water into a fine paste, and by boiling with water a sulpho-arsenite of barium is obtained; to this is added dilute sulphuric acid which precipitates a mixture of barium sulphate and sulphide of arsenic as a fine yellow colour, which is collected, washed, and dried.

2. By Sublimation.-1 lb. of sublimed sulphur and 2 lbs. of white arsenic are thoroughly mixed together and placed in a crucible; this is covered with another crucible or with a special condenser, and heated in a furnace. The arsenic and sulphur react and form the sulphide, which, being volatile, sublimes into the cover, and is collected, washed, and dried; it varies a little from time to time in shade.

King's or arsenic yellow is a very bright pigment, almost rivalling the chromes in beauty. It has good body and works well either in oil or water, but is not a durable colour, as exposure to light causes it to fade, while air and moisture have no action on it. It does not mix well with other pigments, since, when associated with lead pigments, or with verdigris, emerald green, or other copper pigments, it gradually acquires a dark brownish tint owing to the formation of the black sulphides of lead and copper. It can be mixed with ultramarine, cadmium yellow, or oxide of iron without change. Being an arsenic colour it is very poisonous, and, therefore, its use is not to be recommended. Partly in consequence of this objection it has become nearly, if not entirely, obsolete. King's yellow forms a colourless solution with strong hydrochloric acid; as also with caustic soda, from which, on adding acid, the pigment is reprecipitated. The presence of arsenic may be tested for by means of Marsh's test, described in the section on emerald green (p. 171).

REALGAR, ARSENIC ORANGE.

Realgar is a native arsenic disulphide found in small quantities. in various localities. It is prepared artificially by a process of sublimation as follows:-(a) A mixture of 8 lbs. of white arsenic (arsenious oxide) and 4 lbs. of flowers of sulphur are heated in a crucible as in making orpiment. (b) A mixture of 30 lbs. of arsenic, 20 lbs. of flowers of sulphur, and 40 lbs. of charcoal is made; a charge of 60 lbs. of this mixture is heated at a time in earthenware crucibles so arranged that the product which sublimes can be collected. This sublimate is then remelted to form the colour. Realgar has the same properties as orpiment.

INDIAN YELLOW.

Indian yellow or Purree is a most curious product. It has long been used in India, but is of comparatively recent introduction in this country, where its use is limited. It is made exclusively at Monghyr in Bengal by a caste of people known as the Gwalas. It is made from the urine of cows fed upon the leaves of the mango tree, which food increases the secretion of bile and the excess passes into the urine to which it imparts a strong yellow colour. The flow of the urine is stimulated by the Gwalas gently rubbing the urinary organs two or three times a day; indeed, the cows are so habituated to this that they are unable to pass the urine themselves; the feeding with mango leaves is so injurious that its long continuance causes the death of the cows, and grass, &c., is occasionally substituted for them; the average life of these cows is from six to seven years. The urine as it comes from the cows is collected, and each evening it is boiled down in earthen vessels when the yellow is deposited; it is gathered on calico, made into balls and sent into the market for sale. The annual production is said to be about 100 to 120 cwts. Indian yellow is a fairly bright yellow pigment, and is sold in the form of small round balls; it is non-poisonous and has a good colouring power; unfortunately it is not durable, as exposure to light soon causes it to fade. Authorities differ somewhat upon the composition of Indian yellow, but most agree that it is a compound of a peculiar acid known as euxanthic acid (which exists in the purree) combined with magnesia; there is also present potassium benzoate and other bodies. The acid itself generally crystallises in small needles of a pale yellow colour; it is slightly soluble in cold water, more readily in boiling

water, and freely in ether and alcohol. On being subjected to dry distillation a yellow body, called euxanthone, sublimes. The salts of euxanthic acid are all yellow-coloured bodies; those of the alkalies are soluble in water, while those of most of the metals are insoluble and may, therefore, be used as pigments.

CADMIUM YELLOW.

This important yellow pigment, so much used by artists on account of the brilliance of its colour and its permanence, is the sulphide of the metal cadmium and is composed of

and has the formula Cd S. It is made by passing a current of sulphuretted hydrogen gas through solutions of cadmium salts, as shown in the equation :

*

G. Buchner has investigated the properties of cadmium yellow more thoroughly than any other chemist. He describes four modifications of cadmium sulphide which he distinguishes as

1. A-modification, obtained by passing a current of sulphuretted hydrogen gas through a slightly acid solution of a cadmium salt. The colour is a very bright and pure citron-yellow, has a good body, and works well as an oil-colour. By various means it can be converted into the B-modification. When used as an oil-colour it is quite permanent, but when used in water, or kept in a moist atmosphere, it gradually undergoes oxidation, passing into sulphate, this change being accompanied by a loss of colour.

2. B-modification.-This has a bright vermilion-red colour, and is obtained by passing sulphuretted hydrogen gas through a strongly acid solution of a cadmium salt. It is the most permanent form of cadmium sulphide and is unaffected by exposure to light and air.

The author has been unable to prepare this red variety of cadmium yellow. Although Buchner does not give any clear description of the method of obtaining it, yet from the remarks as to the conditions under which this red variety is formed, it is evident that it cannot be obtained by precipitation free from the yellow variety, and that the process of separation consists in * Chemiker Zeitung quoted in Journal Socy. Chem. Ind., VI. 665.

exposing the mixture of yellow and red sulphides to light and air for some considerable period. It is not practicable to prepare this variety as a commercial article.

3. C-modification.-A variety soluble in water, of no practical interest, and prepared by a process of dialysis.

4. D-modification.-This variety has a pale yellowish colour and little or no body. It is prepared by passing sulphuretted hydrogen gas through an ammoniacal solution of a cadmium salt. It is of no practical use.

Cadmium yellow is made commercially in various shades of yellow and orange, the processes for the production of which are

described below.

Preparation of Yellow Cadmium.-This is prepared in several ways. 1. A slightly acid solution of any cadmium salt is prepared and through it is passed a current of sulphuretted hydrogen gas; the apparatus shown in Fig. 17, p. 111 may be used. This has a pure chrome-yellow shade. 2. A lemon-yellow shade is obtained by dissolving 1 lb. of cadmium sulphate in 4 gallons of water and adding 14 gallons of the ordinary yellow ammonium sulphide. 3. Or a solution of cadmium sulphate is made, to which is added a solution of sodium thiosulphate and a little sulphuric acid and the mixture boiled for an hour. This variety contains much free sulphur, and is, hence, liable to undergo oxidation to sulphuric acid, which destroys the yellow.

Preparation of Orange Cadmium.-1. A solution of cadmium sulphate or chloride is prepared. It is made strongly acid by the addition of excess of hydrochloric acid, and a current of sulphuretted hydrogen gas is passed through it. 2. 1 lb. of cadmium sulphate is dissolved in 4 gallons of water; the solution is boiled, and while boiling, yellow ammonium sulphide is added. All the precipitates of yellow obtained in the various ways just described must be well washed in water, especially those obtained with the ammonium sulphide, to free them from any trace of acid or sulphide which, if left in, would ultimately lead to the destruction of the colour. After being washed they should be thoroughly dried at as low a temperature as possible, not exceeding about 150° to 160° F.; too high a temperature causes the shade to become brown while hot and although the colour comes back on cooling, yet it never quite regains the original brilliancy.

PROPERTIES OF CADMIUM YELLOW.-Pure cadmium yellow is one of the most permanent pigments known; it is unaffected by exposure to light and air. It mixes with any vehicle used in painting. When heated strongly the colour darkens, changing to a dark violet-red; on cooling, the original