WHITE LEAD 1133 



of boiling to dryness, I merely heat the solution for some time to the boiling 

 point, by which the excess of carbonic acid is partly driven off, and pure carbonate 

 of magnesia is precipitated, -which may then be collected, and dried in the same way 

 as if precipitated by a carbonated alkali. If I require sulphate of magnesia, I 

 neutralise the solution of bicarbonate of magnesia with sulphuric acid, boil down, 

 and crystallise ; or I mix the solution with its equivalent quantity of sulphate of iron, 

 dissolved in water, heated to the boiling point, and then suffer the precipitated car- 

 bonate of iron to subside ; after which I decant the clear solution of sulphate of 

 magnesia, boil down, and crystallise as before. When using this solution of bicar- 

 bonate of magnesia for the purpose of preparing carbonate of lead, I make a saturated 

 solution of chloride of lead in water, which at a temperature of 50 or 60 Fahr., 

 has a specific gravity of about 1*008, and consists of 1 part of chloride of lead dis- 

 solved in 126 parts of water. I then mix the two solutions together, when carbonate 

 of lead is immediately precipitated ; but in this operation I find it necessary to use 

 certain precautions, otherwise a considerable quantity of chloride of lead is carried 

 down along with the carbonate. These precautions are, first, to use an excess of the 

 solution of magnesia ; and secondly, to mix the two solutions together as rapidly as 

 possible. As to the first, when using a magnesian solution containing 1,600 grs. 

 of carbonate of magnesia, per imperial gallon, with a solution of chloride of lead 

 saturated at 55 or 60 Fahr., 1 measure of the former to 8.J of the latter is a proper 

 proportion ; in which case there is an excess of carbonate of magnesia employed, 

 amounting to about an eighth of the total quantity contained in the solution. When 

 either one or both the solutions vary in strength, the proportions in which they are to 

 be mixed must be determined by preliminary trials. It is not, however, necessary to 

 be very exact, provided there is always an excess of carbonate of magnesia amount- 

 ing to from one-eighth to one-twelfth of the total quantity employed. If the excess 

 is greater than one-eighth, no injury will result, except the unnecessary expenditure 

 of the magnesian solution. As to the second precaution, of mixing the two solutions 

 rapidly together, it may be accomplished variously ; but I have found it a good 

 method to run them in two streams, properly regulated in quantity, into a small 

 cistern, in which they are to be rapidly blended together by brisk stirring, before 

 passing out, through a hole in the bottom, to a large cistern or tank, where the 

 precipitate finally settles. The precipitate thus obtained is to be collected, washed 

 and dried in the usual manner. It is a carbonate of lead, very nearly pure, and 

 suitable for most purposes ; but it always contains a small portion of chloride of 

 lead, seldom less than from 1 to 2 per cent., the presence of which, even in so small 

 a quantity, is somewhat injurious to the colour and body of the white lead. I decom- 

 pose this chloride, and convert it into a hydrated oxide of lead by grinding the dry 

 precipitate with a solution of caustic alkali, in a mill similar to the ordinary mill 

 used in grinding white lead with oil, adding just so much of the lye as may be 

 required to convert the precipitate into a soft paste. I allow this paste to lie a 

 few days, after which, the chloride of lead being entirely, or almost entirely, decom- 

 posed, I wash out the alkaline chloride formed by the reaction, and obtain a white 

 lead, similar in composition to the best white lead of commerce. I prepare the 

 caustic alkaline lye by boiling together, in a leaden vessel, for an hour or two, 1 part 

 by weight of dry and recently-slaked lime, 2 parts of crystallised carbonate of soda 

 (which being cheaper than carbonate of potash, I prefer), and 8 parts of water. The 

 clear and colourless caustic lye, obtained after subsidence, will have a specific gravity 

 of about 1-090, and when drawn off from the sediment, must be kept in a close vessel 

 for use.' 



More recently Mr. Peter Spence, of Manchester, has patented a process for obtaining 

 white lead directly from the ores. 



As we have before stated, the manufacture of white lead by the Dutch process is 

 one the nature of which seems yet enveloped in considerable obscurity. So far as ap- 

 pearances go, the action would seem to consist: first, in the oxidation of metallic lead 

 by the atmosphere, under the influence of the vapoiir of acetic acid ; secondly, in the 

 production of acetate of lead, by the combination of the oxide of lead with the acetic 

 acid ; and thirdly, in the displacement of the acetic acid from its union with the 

 oxide of lead, by the action of carbonic acid, and the consequent formation of white 

 lead. But this in no way accounts for the fact, that, when acetate of lead is decom- 

 posed by carbonic acid, it is carbonate of lead, and not white lead, which is formed. 

 Nor can we conceive how an acid like the acetic is capable of being wholly expelled 

 from a metallic oxide by a quantity of another acid incapable of completely saturating 

 the oxide. In other words, as white lead contains free or uncombined oxide of lead, 

 how happens it that the free acetic acid does not remain united to this ? We confess 

 our inability to reconcile the facts of the case with the preceding hypothesis, and 

 therefore -pass on to another, in which we will assume that acetate of lead, but not the 



