Black Finish for Brass.― Make a strong solution of nitrate of silver in one dish and nitrate of copper in another. Mix the two together and plunge the brass into it. Now heat the brass evenly until the required degree of dead blackness is obtained. This is the method used by French instrument-makers to produce the beautiful dead-black color so much admired in optical instruments. Lacquer for Brass Castings.-Take of shellac, 6 oz.; amber of copal, ground, 2 oz.; dragon's blood, 40 grains; extract of red sandal-wood, 30 grains; oriental saffron, 36 grains; pounded glass, 4 oz.; very pure alcohol, 44 oz. To apply to brass, expose to a gentle heat and dip them in. Solder. The following solder will braze steel or iron, and may be found very useful in case of a valve-stem or other light portion of an engine or machine breaking at a time when it is important that the engine or machine should continue work: Silver, 19 parts; copper, 1 part; brass, 2 parts. Fusible Metal, consisting of 8 parts of bismuth, 5 of lead, and 3 of tin. It melts at the heat of boiling water, or 212° Fah. By the addition of a very little mercury, it becomes still more fusible, and is used for certain anatomical injections and for the filling of carious teeth. Rule for finding the approximate weight of iron castings from patterns. Multiply the weight of the pattern by the figures corresponding to the material in the table. Very accurate results cannot be expected, as the specific gravity of wood as well as of iron varies. TABLE SHOWING THE WEIGHT OF CASTINGS BY WEIGHT OF THE PATTERNS. Multiply the weight of the pattern by the multiplier opposite SHOWING THE SHRINKAGE OF CASTINGS OF DIFFERENT METALS. Cast-iron, inch per lineal foot. Tin, inch per lineal foot. || 1⁄2 16 TABLE SHOWING THE WEIGHT AND BULK OF DIFFERENT SUBSTANCES IN CUBIC FEET, POUNDS AND TONS. TABLE SHOWING THE WEIGHT OF DIFFERENT METALS PER CUBIC FOOT. SHOWING THE ACTUAL EXTENSION OF WROUGHT-IRON AT VARIOUS 392 .......... .1.0025757 672.1.0043253 752 ..... 1.0063894 Surface becomes Surface becomes straw-colored, deepyellow, crimson, violet, purple, deepblue, bright-purple. 932 ..... .1.0087730) Surface becomes dull, and then bright 1,112 ...... ..1.0114811 S red. .1.0216024 ...... 1,652 ... 2,192 ... .1.0348242 2,732 1.0512815 Bright-red, yellow, welding heat, white 2,912 .... Linear Expansion of Wrought-Iron. The linear expansion which a bar of wrought-iron undergoes, according to Daniell's pyrometer, when heated from the freezing- to the boiling-point, or from 32° to 212° Fah., is about of its length; at higher temperatures the elongation becomes more rapid. Thus, it will be seen how sensible a change takes place when iron undergoes a variation of temperature. A bar of iron 10 feet long, subject to an ordinary change of temperature of from 32° to 180° Fah., will elongate more than of an inch, or sufficient to cause fracture in stone-work, strip the thread of a screw, or endanger a bridge, floor, roof, or truss, or even push out a wall if brought in contact with it. The expansion of volume and surface of wrought-iron is calculated by taking the linear expansion as unity; then, following the geometrical law, the superficial expansion is twice the linear, and the cubical expansion is three times the linear. Wrought-iron will bear on a square inch, without permanent alteration, 17,800 pounds, and an extension in length of 1400. Cohesive force is diminished 3000 by an increase of one degree of heat. Compared with cast-iron, its strength is 1-12 times, its extensibility 0.86 times, and its stiffness 13 times. Cast-iron expands 162000 of its length for one degree of heat; the greatest change in the shade, in this climate, is length; exposed to the sun's rays, 1000 98 of its Cast-iron shrinks, in cooling, from tog of its length. Cast-iron is crushed by a force of 93,000 pounds upon a square inch, and will bear, without permanent alteration, 15,300 pounds upon a square inch. To find the surface dilatation of any particular article, double its linear dilatation; and to find the dilatation in volume, triple it. To find the elongation in linear inches, per linear foot, of any particular article, multiply its respective linear dilatation, as given in the table, by 12. TABLE SHOWING THE LINEAR DILATATION OF SOLIDS BY HEAT. Length which a Bar Heated at 212° has greater than when at the Temperature of 32°. TABLE DUCED FROM EXPERIMENTS ON IRON PLATES FOR STEAM-BOILERS, BY THE FRANKLIN INSTITUTE, PHILADA. on boiler-plate was found to increase in tenacity, as its temture was raised, until it reached a temperature of 550° above freezing-point, at which point its tenacity began to diminish. 32° to 80° tenacity is 56,000 lbs., or below its maximum. the maximum. 570° 720° 55,000 the same nearly as at 30°. 66 66 66,000 66 66 will be seen by the above table that if a boiler should become heated by the accumulation of scale on some of its parts, or nsufficiency of water, the iron would soon become reduced to than one-half its strength. TABLE WING THE STRENGTH OF COPPER BOILER PLATES AT DIFFERENT EMPERATURES, DEDUCED FROM EXPERIMENTS BY THE FRANKLIN INITUTE OF PHILA. THE STANDARD STRENGTH AT 32° BEING 32,800 BS. PER SQUARE INCH. t will be seen from the above table, that, in being heated from freezing-point to the boiling-point of water, copper loses 5 cent. of its strength; at 550° it loses about one-quarter of its ength; and at 1332° loses all its tenacity. |