Scientific American Supplement, No. 520, December 19, 1885 | Page 6

metal of D'Arcet is composed of: bismuth, 8 parts; lead, 5 parts; tin, 3 parts. It melts at 173.3°. We can, therefore, by proper mixture, form a solder which will melt at any desirable temperature. Numerous devices for closing doors automatically have been constructed, all depending upon the use of the fusible solder catch.
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STEEL STRUCTURES.
At a recent meeting of the Engineers' Club of Philadelphia, Mr. James Christie presented a paper upon "The Adaptation of Steel to Structural Work." The price of steel has now fallen so low, as compared with iron, that its increased use will be actively stimulated as the building industries revive. The grades and properties of the steels are so distinct and various that opinions differ much as to the adaptability of each grade for a special purpose. Hitherto, engineers have favored open hearth steel on account of uniformity, but recent results obtained from Bessemer steel tend to place either make on equality. The seeming tendency is to specify what the physical properties shall be, and not how the steel shall be made.
For boiler and ship plates, the mildest and most ductile steel is favored. For ships' frames and beams, a harder steel, up to 75,000 pounds tenacity, is frequently used. For tension members of bridges, steel of 65,000 to 75,000 pounds tenacity is usually specified; and for compression members, 80,000 to 90,000 pounds. In the Forth Bridge, compression steel is limited to 75,000 to 82,000 pounds. Such a marked advantage occurs from the use of high tension steel in compression members, and the danger of sudden failure of a properly made strut is so little, that future practice will favor the use of hard steel in compression, unless the material should prove untrustworthy. In columns, even as long as forty diameters, steel of 90,000 pounds tenacity will exceed the mildest steel 35 per cent., or iron 50 per cent., in compressive resistance.
The present uncertainty consists largely as to how high-tension steel will endure the manipulation usual with iron without injury. A few experiments were recently made by the writer on riveted struts of both mild and hard steel, which had been punched, straightened, and riveted, as usual with iron, but no indication of deterioration was found.
Steel castings are now made entirely trustworthy for tensile working stresses of 10,000 to 15,000 pounds per square inch. In some portable machinery, an intermittent tensile stress is applied of 15,000 pounds, sometimes rising to 20,000 pounds per square inch of section, without any evidence of weakness.
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Equal volumes of amyl alcohol (rectified fusel oil) and pure concentrated hydrochloric acid, shaken together in a test tube, unite to form a single colorless liquid; if one volume of benzine (from petroleum) be added to this, and the tube well shaken, the contents will soon separate into three distinct colorless fluids, the planes of demarkation being clearly discernible by transmitted light. Drop into the tube a particle of "acid magenta;" after again shaking the liquids together, the lower two zones will present different shades of red, while the supernatant hydrocarbon will remain without color.
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A METHOD OF MEASURING THE ABSOLUTE SENSITIVENESS OF PHOTOGRAPHIC DRY PLATES.
[Footnote: From the Proceedings of the Academy of Arts and Sciences.--_Amer. Jour._]
By WILLIAM H. PICKERING.
Within the last few years the subject of dry plate photography has Increased very rapidly, not only in general popularity, but also in importance in regard to its applications to other departments of science. Numerous plate manufacturers have sprung up in this country as well as abroad, and each naturally claims all the good qualities for his own plates. It therefore seemed desirable that some tests should be made which would determine definitely the validity of these claims, and that they should be made in such a manner that other persons using instruments similarly constructed would be able to obtain the same results.
Perhaps the most important tests needed are in regard to the sensitiveness of the plates. Most plate makers use the wet plates as their standard, giving the sensitiveness of the dry plates at from two to sixty times greater; but as wet plates vary quite as much as dry ones, depending on the collodion, condition of the bath, etc., this system is very unsatisfactory. Another method, employed largely in England, depends on the use of the Warnerke sensitometer. In this instrument the light from a tablet coated with luminous paint just after being exposed to a magnesium light is permitted to shine through a colored transparent film of graduated density upon the plate to be tested. Each degree on the film has a number, and, after a given exposure, the last number photographed on the plate represents the sensitiveness on an empirical scale. There are two or three objections to this instrument. In