Vol. XXII. No. 1.] 



POPULAR SCIENCE NEWS. 



5 



Prattital €l)emt0trp ant tl)t Strtct. 



WHITE-LEAD. 



This very important salt of lead, which is so 

 extensively used as a paint, is, chemically con- 

 sidered, a basic carbonate. Thus, to the regu- 

 lar neutral carbonate Pb O CO^, we may add ad- 

 ditional atoms of plumbic oxide, forming a com- 

 pound with the general symbol (PbO)xC02; 

 and the covering power (and consequently the 

 value of the product) seems to depend upon 

 the number of additional molecules of oxide 

 that we can attach to a molecule of carbonic 

 dioxide. 



Although numerous processes for the scien- 

 tific manufacture of white-lead have been 

 invented, none of them are (juite as satisfac- 

 tory as the old-fashioned "Dutch" method, 

 and probablj- more than nine-tenths of all the 

 lead in the market is made by that process. 

 It was realh" introduced into Europe bj- the 

 Saracens, who carried on the manufacture in 

 Southern Spain, whence the Dutch brought 

 it to Holland in the sixteenth century. In 

 this process, the lead, which is first cast into 

 perforated disks called "buckles," is placed 

 in earthen pots in the bottom of which is a 

 little vinegar. The pots are then covered, 

 and buried iu a mass of fermenting stable 

 manure, to which spent tan is sometimes 

 added. Owing to the heat of the fermenting 

 manure, the vinegar is evaporated, and the 

 acid vapors combine with the metal to form 

 the basic acetate of lead, which in turn is 

 converted into the basic carbonate by the 

 action of the carbonic acid, which is also a 

 product of the fermentation. In a few weeks 

 the pile is opened, the pots removed, and the 

 white-lead separated from the pieces of unal- 

 tered metal which remain. It is then usually 

 ground in oil, and comes into the market in 

 the familiar form of a pasty mass. 



Attempts have been made to substitute, iu 

 the place of the unreliable manure-heap, a 

 closed box or chamber in which the lead is 

 placed, and into which vapors of acetic acid 

 and carbonic-acid gas are passed. It is also 

 made in the wet vfa.y by dissolving litharge 

 (oxide of lead) in acetic acid, thus obtaining 

 a solution of basic acetate of lead, and then 

 passing a current of carbonic-acid gas through 

 the solution, which precipitates part of the 

 lead as a basic carbonate, leaving a neutral 

 acetate in solution ; more litharge is then 

 added to the solution of neutral acetate, and 

 the process repeated. 



For some unknown reason, none of these 

 processes give quite as good a product as the 

 original Dutch method ; and it seems to be one 

 of those rare cases where " the rule of thumb " 

 is more successful than a more rational and 

 scientific method. Possibly one reason is, the 

 difficulty of obtaining a supply of pure car- 

 bonic-acid gas. It is usually obtained from 

 the combustion of coke or coal, and is likely 

 to contain traces of sulphur compounds, which 

 discolor the white-lead by uniting with it to 

 form the black sulphide. At Brohl, in Ger- 



many, there is a natural supply of the gas 

 from the earth, which is used for this purpose. 



Silver, which is usually a very desirable 

 metal, is a source of annoyance to the manu- 

 facturers of white- lead. Lead frequently con- 

 tains a small percentage of the precious metal ; 

 and,- if the quantity is appreciable, it spoils 

 the color of the finished product, owing, prob- 

 ablj-, to the well-known blackening eflfect of 

 light upon silver salts. There are numerous 

 other precautions which must be taken to 

 obtain a first-class article of white-lead, and 

 the necessary- knowledge can only be gained 

 by long experience. 



The manufacture of white-lead is properlj- 

 considered as a "death-dealing trade," more 

 especially to the workmen who have charge of 

 the grinding of the dry powder. The fine 

 lead-dust is absorbed into the system, and 

 produces a train of very characteristic symp- 

 toms due to lead-poisoning. In France the 

 paternal government of that country requires 

 manufacturers of white-lead to pension their 

 employes after a certain age, considering 

 that their health will be so impaired that they 

 will be unable to earn their living. Once, 

 while visiting a manufactorj-, we noticed that 

 some facetious person had inscribed upon the 

 grinding-machine the rather startling legend, 

 "death's friend." It is said, however, that, 

 with due care, no cases of poisoning need 

 occur, and that the evil eflfects are largely due 

 to the carelessness of the workmen, who neg- 

 lect the necessary precautions of cleanliness, 

 etc. The phj-sician in charge of a large city 

 hospital where cases of lead-poisoning are fre- 

 quently treated, informs us, that, after being 

 once cured, the patients return to their old 

 trade, and are soon back in the hospital suf- 

 fering from the same disease, — a curious 

 example of the force of habit. 



The liability of white-lead to be discolored 

 bj' sulphuretted hydrogen renders it an unsuit- 

 able paint for localities where the air is likely 

 to be contaminated by sulphur compounds. 

 Some years ago the paint on some buildings 

 near the mouth of a sewer near this city was 

 turned nearly black by the gases arising from 

 it. This discoloration was intended to be 

 brought forward in a lawsuit as evidence of 

 the nuisance created bj- certain manufactories 

 which were drained by the sewer. Before the 

 case came to trial, however, the ozone in the 

 sea-air had oxidized the black sulphide to 

 the white sulphate, and thus the evidence was 

 destroyed by natural causes. In the case of 

 old paintings where the white tones have 

 become discolored, it has been recommended 

 to apply a solution of peroxide of hydrogen, 

 which will have a similar effect, and thus 

 restore the brilliancy of the picture. 



LABORATORY TABLES. 



Though it has its drawbacks, there can be little 

 doubt that the best material for a laboratory table- 

 top is wood. Other materials may be better for 

 special purposes, but for general use wood is best; 

 and among woods teak is to be preferred. The 

 wood ought never to be varnished or painted, as the 



action of alkalies and strong acids on the prepara- 

 tions employed renders the table unsightly. The 

 now common custom of saturating the wood with 

 melted parafHne is not open to the same objection, 

 and has, indeed, many advantages. The parafRne 

 renders the wood impervious to water, and, on ac- 

 count of its chemical indifference, it prevents the 

 table being seriously attacked by sulphuric acid or 

 caustic soda, which will sometimes be spilled even 

 in the best-regulated laboratories. Probably the 

 chief objection to paratfine is, that it melts and 

 adheres to any warm vessel or apparatus that may 

 chance to come in contact with it. In this way a 

 new source of error is introduced into analyses, as 

 the slight coating of para/Bne on a capsule or cru- 

 cible lid, being invisible, may be forgotten to be 

 removed by ignition or otherwise before weighing. 

 Of course, it is easy to say, " Never lay any vessel 

 you intend to weigh on the table; " but a possible 

 source of error is present, all the same. My own 

 view of the matter is, that for qualitative and 

 general work it is well to have the tables paraffined, 

 while for quantitative and finer work it is better to 

 have the surface clean. 



Slate is objectionable as a material for tables, 

 firstly, because of its antipathy to glass vessels; 

 secondly, because it is not readily pierced or cut; 

 and thirdly, because it is cold to the touch and 

 cold to look at. Yet it is the best kind of surface 

 on which to place the combustion-furnace, air-bath, 

 and similar apparatus. In like manner, although 

 lead is not to be commended for general use, it is 

 a good plan to have one table covered with it, and 

 so arranged that any liquid falling thereon may 

 collect at one end, and so be run off into the waste. 

 All distillations should be conducted on this table, 

 which is not only a convenient arrangement for 

 getting rid of the water used in condensing, but is 

 also a security against danger when working with 

 inflammatory liquids. 



The attempt to convert each working bench into 

 a sort of complete laboratory seems to be a mis- 

 take, however heretical this view may appear to 

 some. To have tank-water, distilled water, steam, 

 gas, draught, and sink " laid on " at each bench, 

 produces an unnecessary and expensive complica- 

 tion of pipes; and the more pipes and stopcocks, 

 the more plumbers and repairs. Simplicity is a 

 great principle in the arrangement of laboratories, 

 as in most other matters. All materials and appa- 

 ratus required by each worker should be, within 

 easy reach, but not necessarily at his own particu- 

 lar table, which should, indeed, only be supplied 

 with those requisites the absence of which would 

 produce positive inconvenience. — D. B. Scott, in 

 Chemist and Druggist. 



THE INFLUENCE OF SILICON ON THE 

 PROPERTIES OF STEEL. 



The following are the general conclusions arrived 

 at by a committee of the British Association from 

 a series of experiments conducted by them. On 

 adding silicon, in the form of silicon pig, to the 

 purest Bessemer iron, the following results are 

 obtained : — 



The metal is quiet in the mould when even a few 

 hundredths per cent of silicon is added. The 

 metal is originally red-short, especially at a dull red 

 heat, though it works well at a welding tempera- 

 ture; the red-shortness is increased by silicon. In 

 all cases examined, the metal was tough cold and 

 welded well, silicon having little or no influence. 

 Silicon increases the elastic limit and tensile 

 strength, but diminishes the elongation and the 

 contraction of area, a few hundredths per cent 

 having a remarkable influence in this respect. 

 The appearance on fracture by tensile force is 



