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

partitions into compartments; the partitions are so constructed that each alternate one does not quite reach the top while the others do not quite reach the bottom, as shown in the figure ; the object of this is to make the carbonic acid gas, which is sent into the chamber at one end, take a circuitous course before it passes out at the other end. Above the chamber is a tank, the bottom of which forms the roof of the chamber, which bottom is perforated with a large number of fine holes, so that any liquor which may be run into the tank flows through into the chamber below, in a fine stream like rain. In another tank a solution of basic acetate of lead is prepared in the usual way, this flows into the chamber tank and from thence into the chamber; here it comes into contact with carbonic acid gas which is sent into the chamber, the action between the lead solution and the gas being facilitated by the liquor being in such a finely divided form.

The lead solution falls down to the bottom of the chamber, and thence into settling tanks, where the white lead which is formed settles; it is collected, washed, dried, and finished in the usual way, while the solution of neutral acetate of lead, which is also obtained, is used over again.

3. Milner Process.-Milner does not use the basic acetate of lead in his process, but prepares his lead solution by taking 4 lbs. of finely-ground litharge, and mixing it with 1 lb. of salt dissolved in 16 lbs. of water, the mixture being made in wooden tanks. The patentee states that these should be made of yellow pine; oak-wood tanks will not do. In the tanks the mixture is well agitated for about 4 hours, at the end of which time it will have been converted into the basic chloride of lead. When the basic chloride has been fully formed, it is run into covered wooden tanks fitted with agitators; through these tanks a current of carbonic acid gas passes, which, acting on the basic chloride, converts the latter into white lead. Instead of this procedure, the basic chloride may be mixed in lead-lined tanks with caustic soda, and gas is passed into the tanks, as before, until the liquor ceases to be alkaline. This point is ascertained by the workmen taking a little of the mixture out of the tanks from time to time; if it appear viscid, forming a homogeneous mass and an even layer on the sides of the glass, then sufficient gas has not been passed in; if, however, it forms a sort of arborescent pattern on the sides of the glass, the operation is finished; the current of gas is then stopped, and the white lead sent to be finished in the usual way.

The process is said to yield a white lead of good colour and body, and very heavy, weighing about 200 lbs. to a cubic foot.

It was worked by the Sankey White Lead Co., but has been discontinued for some time.

4. Martin Process.-Martin's process for the preparation of white lead is based on the action of carbonic acid on solutions of basic acetate of lead; whether this process was ever used on the large scale the author has no knowledge. One great fault of all precipitation-processes for the manufacture of white lead is that they are apt to give a product which is more or less crystalline, a condition fatal to its being of good quality; the colour may be good, but the body is always deficient. The patentee states that this depends upon the proportion of acid solvent of the litharge to the water which is used in the process; if the water be in excess, then too much basic salt is formed, and the carbonic acid, acting too energetically upon this, causes the formation of a crystalline product; therefore the acid solvent must be in excess. Martin prepares a solution of the neutral acetate of lead in one and a-half times its weight of water, or litharge may be dissolved in acetic acid in such a way as to produce a similar solution. 3,600 gallons of this solution are placed in a tank fitted with an agitator; there is then added 4 to 6 tons of granulated lead, and half a ton of litharge. After thoroughly mixing the materials together, carbonic acid gas is passed in for an hour, when all the litharge will have been converted into white lead, then half a ton more litharge is added, and more carbonic acid; in about an hour this second lot of litharge will be converted into white lead, then more litharge is added, carbonic acid being meanwhile sent in; these additions of litharge are continued hourly until sufficient white lead has been formed, when it is collected and finished in the usual way. If thought desirable, instead of adding the litharge in lots every hour it may be run in in a constant stream. During the operation the temperature is maintained at about 100° F.

The distinctive feature of this process is using the litharge in an undissolved form, and strong solutions of lead acetate. In the absence of practical experience of the process it is not easy to speak definitely on the effect of using such strong solutions; but, judging from the known effects of using strong solutions on the character of precipitates obtained in other cases, one would naturally imagine that the white lead formed would have a crystalline character, and not that amorphous condition which is required in good white lead, still the patentee states that such is not the case.

When the principles which underlie these precipitationprocesses are considered, it becomes evident that the character

of the white lead, both chemically and physically, materially affects its value as a pigment. This character will depend upon the character of the solution of lead which is used, the temperature at which the reaction between the carbonic acid gas and the lead salt takes place, and the strength of the solutions used; on these points information is scanty, and very few of the inventors of white-lead processes have mentioned the influence of any of them. The character and basicity of

the lead salt will have some influence on the result; the basicity should be due to the presence of lead hydroxide, and not to lead oxide, or, at all events, the latter should be present in only small quantities. To ensure the production of lead hydroxide, water seems to be necessary, and therefore should be used in sufficient quantity. The quantity of carbonic acid should be so regulated that not more than two-thirds of the base present is converted into carbonate; if too much gas is used, then all the base will be liable to be converted into carbonate, and the white lead has a tendency to become crystalline; the difficulty is to ascertain when sufficient gas has been used. The strength of the solution of lead will also have some influence, but the diversity of opinion among whitelead makers as to the proper strength is great; some prefer strong solutions, others weak ones. As a rule, weak solutions give the finest precipitates, and strong solutions give the coarsest. The temperature at which the operation is conducted will have some little influence; cold solutions will cause the formation of fine precipitates, while hot solutions tend to give rise to crystalline precipitates, due to the fact that the reaction between the carbonic acid and the lead salt takes place too readily; still it is not desirable to work with solutions that are too cold; the best temperature is from 100° to 120° F.

5. Fourmentin Process.-This was proposed many years ago, and somewhat resembles Milner's process. Litharge is taken and treated with salt in such proportions as to convert it into oxychloride of lead; this body is placed along with water in a number of cylindrical vessels fitted with radial beaters. Carbonic acid is sent in, while the temperature is maintained at the boiling point. When the reaction between the acid and the lead has finished, the current of gas is stopped and the product run into a boiler, in which it is boiled with a quantity of finely-powdered carbonate of lime, equivalent to the amount of salt used in the preparation of the oxychloride, this boiling being continued until, on taking out a sample, and filtering off and testing the clear liquor with ammonia and