612 COLORING OF METALS. 



the navigable channel, the depth of the girders being correspondingly de- 

 creased, the minimum being 12 feet. To provide for expansion by heat, 

 which Avill amount to something like seven feet in the whole length of the 

 bridge, the girders have been adjusted to the piers to allow a certain amount 

 of free play. The bridge has been calculated for a rolling load of one and 

 a half tons to the foot run. Such a burden is more than could be brought 

 upon an}' span by filling its whole length with loaded freight cars ; no part 

 of the bridge would have to undei"go a strain of more than four tons to 

 the square inch, and when it is borne in mind that the iron is actually ca- 

 pable of sustaining a strain of twenty-one tons to the square inch, it will 

 be seen that there is an ample margin of surplus strength. 



COLORING OF METALS. 



Metals may be colored quickly and cheaply by forming on their sur- 

 face a coating of a thin film of a sulphide. In five minutes brass articles 

 may be coated with any color, varying from gold to copper red, then to car- 

 mine, dark red, and from light aniline blue to a blue-white, like sulphide of 

 lead, and at last a reddish white, according to the thickness of the coat, 

 which depends on the length of time the metal remains in the solution 

 used. The colors possess a very good lustre, and if the articles to be col 

 ored have been previously thoroughlj- cleaned by means of acids and alka- 

 lies, they adhere so firmly that they may be op&rated ujjon by the polish- 

 ing steel. To prepare the solution, dissolve one half ounce of hyposulphite 

 of soda in one pound ot water, and add one half ounce of acetate of lead 

 dissolved in half pound of water. When this clear solution is heated to 

 from 190 deg. to 200 deg. Fah., it decomposes slowly, and precipitates sul- 

 phide of lead in brown flakes. If metal be now present, a part of the sul- 

 phide of lead is deposited thereon, and, according to the thickness of the 

 deposited sulphide of lead, the above colors are produced. To produce an 

 even coloring, the articles must be evenly heated. Iron treated with thiy 

 solution takes a steel-blue color ; zinc, a brown color; in the case of copper 

 objects, the first gold color does not appear ; lead and zinc are entirely in- 

 different. If, instead of the acetate of lead, an equal weight of sulphuric 

 acid is added to the hj-posulphite of soda, and the process carried on as be- 

 fore, the brass is covered with a very beautiful red, which is followed by a 

 green (which is not in the first scale of colors mentioned above) and changes 

 finally to a splendid brown with green and red iris glitter. This last is. 

 according co the American Art JournaU a very durable coating, and may 

 find special attention in the manufactures, especially as some of the others 

 are not very permanent. Yery beautiful marble designs can be produced 

 by using a lead solution, thickened with gum tragacanth on brass which has 

 been heated to 210 deg. Far., and is afterwards treated by the nsual solu- 

 tion of sulphide of lead. The solution may be nsed several times. — Van 

 Nostrand's Engineering Magazine. 



