although this may not accurately represent the action which goes on in the majority of cases.

Various salts of lead are used. The differences between the various processes based on the principle just described, depend upon the kind of salt used, and the method of carrying out the operation.

These processes were introduced in the early part of the century, the first patent being dated 1808, and granted to E. Noble. The process described consisted in passing a current of carbonic acid through a solution of lead acetate. A very similar method is known as Thenard's, or the French, process, and will be found described below; while another precipitationmethod is known as the Kremnitz process, having been largely used there for the preparation of white lead.

The precipitation-processes based on the action of carbonic acid gas upon lead salts may be divided into two sub-groups— 3a. Dry methods, in which the lead salt is used in the dry state, or, at the most, simply moistened. 3b. Wet methods, in which, the lead is used in the form of a solution.

3a.-DRY PRECIPITATION PROCESSES.

1. Kremnitz Process.-This process owes its name to having been worked at Kremnitz in Germany. It is carried on in a chamber built of brick or wood, having a number of shelves, on which is placed trays containing a paste made of litharge and either acetic acid or lead acetate, usually in the proportions of 100 lbs. of litharge to 18 pints of acetic acid, or an equivalent quantity of lead acetate solution. When the chamber is filled carbonic acid gas is sent into it, this becomes absorbed by the lead oxide present in the paste, the absorption of the gas being facilitated by raking over the paste from time to time, the mass being kept moist, as this increases the absorption of the gas. The mass originally has a yellowish-grey colour, but as the operation progresses it gradually changes into a white; and when all traces of yellow have disappeared, the operation is stopped, and the white lead which is made is first washed with water, then ground and dried.

Care is taken not to pass the carbonic acid in too long, because this would induce the formation of the normal, instead of the basic, carbonate, which means poor white lead. When carefully worked, good results can be obtained by this process.

The following analysis, presumably of a Kremnitz white lead, is given in Wagner's Technologie :

which shows that this sample did not approach Dutch white lead in composition, but contained more carbonate.

2. Mullin Process.-In this process, which is not now in use, litharge was ground into a paste with water; the paste was then placed in shallow lead-lined boxes, in layers of about an inch and a quarter thick, the boxes were closed by a lid, and then into them was sent currents of carbonic acid and acetic acid gases; the litharge was gradually converted into white lead. The process was, presumably, not a successful one, or it would not have gone out of use.

3b.-WET PRECIPITATION PROCESSES.

In this group of processes for the preparation of white lead, the lead is used in the form of solution, and the precipitation is effected by means of a current of carbonic acid gas. There are a large number of these processes, and many are still in use on the large scale. The differences between the various processes belonging to this group depend upon a variety of circumstances, such as the method of preparing the solution of lead, and the form of apparatus used, on which to a large extent depends the subsidiary, but not unimportant point, the method of applying the carbonic acid to the lead solution.

1. Thenard Process.-This process, from having been worked on a large scale at Clichy, in France, is known as the French process; it is also described in the patent granted to E. Noble in 1808.

The principle of the Thenard process, which is also applicable to many others of this group, is that when a solution of normal lead acetate is boiled with litharge, some of the latter is dissolved, and a solution of basic lead acetate, known as "Goulard's Extract," "Extract of Saturn," &c., is obtained. The reaction is expressed in the following equation:

Pb 2 C2 H3 O2 + 2 PbO + 2H2O = Pb 2 C2 H3 O2, 2 Pb H2 O2

Normal acetate

Litharge.

Water.

Basic acetate of lead.

of lead.

If a current of carbonic acid is passed through this solution of basic acetate of lead, the lead hydroxide it contains is precipitated as a more or less basic carbonate, thus

3 [Pb 2 C2 H3 0, 2 Pb H2 O2] + 4 CO2

=

2 [РЬ СО2, РЬ Н2 02]

White lead.

+ 3 Pb 2 C2 H3 O2 + 4 H2 O

Normal acetate
of lead.

The normal acetate which is thus re-formed can be used again for preparing a fresh solution of basic acetate of lead; of course, while, theoretically, a very little normal acetate is sufficient for the preparation of a large quantity of white lead, and there should be no loss, practically, a small quantity of new acetate has to be added from time to time to make up for the little loss which does occur.

The apparatus used in carrying out the French process at Clichy is shown in Fig. 6. In a vessel, A, of convenient size, litharge is dissolved in a solution of lead acetate, the solution being accelerated by heating the solution by means of the steam pipe, B; from this vessel the liquor in A runs into another vessel, C, in which all insoluble matter settles out. The clear solution is now run into a trough-shaped vessel, D, into which dip a number of pipes connected with the large main pipe, E, through which a stream of carbonic acid gas from the generating system, F G, flows. This system consists of an oven, F, in which is burnt a mixture of chalk and coke, from which a large quantity of carbonic acid gas is evolved; this gas is washed in the apparatus, G, by passing it through water, after which it passes into the solution of lead in the vessel, E, precipitating white lead from it in so doing; the length of time of treating depends upon the quantity and basicity of the lead solution, but usually it takes from 12 to 14 hours. At the end of this time the current of gas is stopped and the white lead allowed to settle; the clear liquor, which is a solution of the neutral acetate, is run into a vessel, H, from which it is pumped up by the pump, I, into the vessel, A, to dissolve more litharge for a fresh operation. The mass of white lead which settles at the bottom of the vessel, E, is run into another vessel, K, from whence it passes on to filters to be washed, and then it is finished in the usual way.

The product given by this process is fairly good, but liable to vary in composition from time to time, according to the strength of the solution of basic lead acetate, and to the basicity or pro

portion of lead oxide the lead acetate has dissolved. These are points to which reference will be made in describing other processes.

2. Cory Process.-The same materials are used in this process as in the last, viz., basic lead acetate and carbonic acid gas, but it differs in the form of apparatus used. The process has been worked on a large scale for a long period.

It was first

Fig. 7.-Cory's process for making white lead.

patented in 1838, and the white lead produced by it is favourably spoken of by users. The author believes that the process

is still in use.

The plant used is shown in Fig. 7. A chamber is built of brickwork; the bottom is made watertight and sloping towards one end so that any liquor which may fall upon it drains away into a tank; this chamber is divided by a number of vertical