differ very much. Under these circumstances it will be best to describe the production of a few of the more prominent shades, and point out how, by various modifications in the recipes, others can be obtained.

All the chrome-yellows are prepared by a comparatively simple and easily-conducted process of precipitation. These colours are admixtures of pure normal lead chromate, with either sulphate of lead (in which case the resulting colour is still called "pure") or with barytes, gypsum, or some other white base.

Sulphate of lead is produced along with the chromate by precipitation, using either sulphate of soda or sulphuric acid as the precipitant, while the barytes, &c., is added to the colour as it is being made.

The method of preparing the chromes is this:-The lead salt, either the acetate or nitrate, is dissolved in a tank, D, Fig. 18, with the necessary quantity of water; heat may be used to facilitate solution, if thought desirable, but the actual precipitation is done cold. In another tank, D, Fig. 18, is dissolved either bichromate of potash or bichromate of soda and sulphate of soda (Glauber's salt). As the commercial products are liable to be dirty, the solutions must be either filtered or allowed to settle in the tanks, and the clear supernatant liquor only used; when both solutions are ready they are run simultaneously into the precipitating tank C, Fig. 18, where the chrome-yellow is precipitated, and is allowed to settle to the bottom of the tank; the clear top liquor is run off by opening one of the plugs, H, fresh water run in, the whole stirred up and again allowed to settle, and the clear liquor run off. The washing is repeated, if thought necessary; for best qualities at least three washings should be given. After the washing the pulpy colour is thrown on to an ordinary felt filter, or put into the filter-press to extract as much water as possible, and then dried in a stove at as low a temperature as possible.

If barytes or gypsum be used to tone down the yellow to the required shade it is put into the precipitating tank C with a quantity of water, with which it is well mixed, so as to get a uniform milky mixture; then the two precipitating solutions, which have been prepared in the tanks D D, are run in, care being taken that the contents of tank C are thoroughly well agitated all the time; afterwards the colour may be finished as before. If the barytes, &c., is not properly mixed there is great liability for it to aggregate in lumps, and the proper shade is not obtained; besides the finished colour will be full of white

masses.

To obtain fine shades it is necessary to keep the lead in excess. Now, theory indicates that for 100 lbs. of lead salt 39 lbs. of potassium bichromate are required; in practice, however, it is found that if the full theoretical amount is used then the shade of the colour is affected, hence it is not advisable to use more than 35 lbs.; the above recipes have been based on this amount, so that in all, the lead is in excess. The weaker the solutions used for precipitating, the finer will the resulting pigment be.

The shade of the pigment obtained depends upon the proportion between the two precipitants, bichrome and Glauber's salt; the more sulphate there is present the paler will be the shade of yellow obtained, a fact which can be gathered by an examination of the above and other recipes (which will be given) for preparing the chrome-yellows. In consequence, the colour-maker can, by altering the proportions of the two bodies, produce almost any shade of yellow he may want. There is a direct connection between the quantities of the two bodies and that of the lead salt used; if the latter be kept constant, then, if the bichrome is increased, the quantity of Glauber's salt must be decreased, and vice versa, so that no loss of material may result in making the chromes.

The following equivalents are given, so that the relative proportions of the two bodies may be kept correctly when the recipes are modified to produce other shades than those given above::

1 lb. of Glauber's salt is equal to 0.4 lb. of bichromate of potash. 1 lb. of bichromate of potash is equal to 2.5 lbs. of Glauber's salt. The difference between the values of acetate and nitrate of lead is so small that in practical use one may be considered equivalent to the other; and the same remark applies to the bichromates of

potash or soda. The above equivalents may be used in this way-Suppose it is desired to make a yellow intermediate in shade between the lemon- and chrome-yellows given above, then 2 lbs. less bichromate must be used than for the chrome-yellow. The quantity of sulphate equivalent to 21 lbs. bichrome required to throw down all the lead is 21 × 261; 61 lbs. of Glauber's salt must, therefore, be added to replace the 2 lbs. of bichrome taken away.

Cologne yellow is the name originally given to yellows containing lead chromate and sulphate, and made by precipitating with a mixture of potassium bichromate and sulphuric acid. Cologne yellows are generally made in two or three shades, pale to deep.

They are made in the same way as the chrome-yellows above described, and are useful to mix with Prussian blue to make greens. American Chrome-Yellow. This is made by using alum in the place of Glauber's salt; the product is a fine one, the alumina salt seeming to have a beneficial influence on the fineness of the precipitate; still care is required in using alum, as otherwise there may be a difficulty in precipitating the pigments. The following recipes will give some idea of the proportions generally used:

Another method of making chromes (which, however, is rarely used) is to take white lead, treat with nitric acid so that part

only of the white lead is dissolved, and then add bichromate of potash; according to the amount of white lead used in relation to the other constituents, so will the shade of yellow be; the following recipes may be taken as examples:

For Chrome-Yellow.—Take 200 lbs. of white lead, mix with water to a fine paste, and then add 40 lbs. of nitric acid of 1.42 specific gravity (84° Tw.); when all effervescence has ceased more water is added, and then a solution of 35 lbs. of bichromate of potash; the yellow formed is finished in the usual way.

For a Deep Lemon-Chrome.-Take 300 lbs. of white lead, treat with water and 40 lbs. of nitric acid, as before, and precipitate with 35 lbs. of bichromate.

A cheaper class of chrome-yellows is made by precipitating the chromate of lead on to a white base; barytes, china clay, and whiting have been used for this purpose. When well made these are quite as good to use as the technically pure chromes described above, although the preference is given to the latter by most users.

The following recipes may be taken as standards for the preparation of these cheap chromes :

The depth of shade may be altered at will by varying the amount of barytes used: an increase in the amount reduces the depth of shade, while a reduction in the amount increases the depth; the quantities of the other materials need not be altered. The method of working has already been given.

China clay can be used instead of barytes, and less weight of it will be required to produce any given shade than of barytes, owing to the china clay being so much lighter in specific gravity,

and, therefore, more bulky weight for weight, the relative power of the two bodies being as 1 to 2 or 3; that is, 1 lb. of china clay will tone down chrome as much as 2 or 3 lbs. of barytes. Gypsum may also be used, and is intermediate between china clay and barytes in its toning powers, but it is not so good for this purpose as the other two bodies, as there is a reaction between the gypsum and lead salts which renders it impossible to keep the lead in excess, and so there is a tendency to change the shade of yellow which is being made; when nitrate of lead is used instead of acetate this action of gypsum is more noticeable.

Whiting is sometimes used to tone down the yellows; for pale shades its use is not advisable, and, although it may be used for deep shades, yet the other whites just named are to be preferred. Being somewhat alkaline in its reactions and liable to contain traces of caustic lime, from being overheated in the process of manufacture, it often has an undesirable tendency to turn the shades of the yellows with which it may be mixed to an orange. When barytes, china clay, gypsum, or whiting are used as toning colours it is advisable to grind them with water to ensure perfect admixture with the other ingredients of the yellow.

2nd. PREPARATION OF CHROME-ORANGES AND SCARLETS.--The affinity between the chromic acid on the one hand and of the lead on the other not being very strong, when the chromates of lead are treated with a stronger base, such as soda or potash, a part of the chromic acid is eliminated, and a basic chromate of lead is formed; this reaction is expressed by the following equation:

The basic lead chromate has a deep red colour; if the action of the alkali only proceeds to a limited extent, then the basic red chromate formed combines with the excess of yellow normal chromate giving rise to an orange-coloured body, which is a mixture of the two chromates of lead; the shade of the orange pigment thus produced will depend, to a large extent, upon the amount of the action which takes place between the normal chromate and the alkali. The preparation of the chrome-oranges and scarlets depends upon the action of alkaline bodies upon the normal lead chromate. As with the yellows no actually chemically-pure oranges are made; the technically-pure colours contain more or less sulphate of lead, while the common colours generally contain barytes. The orange and scarlet chromes are much