Neither of these tests seemed to follow the general formula for = P13 48EI' length; E modulus of elasticity of the metal (taken for aluminum bars as 13,000,000); I= element of inertia of the action. Neither will any other assumed modulus of elasticity for aluminum give results agreeing with the deflection observed under these transverse tests. The flow of aluminum under tensile tests is very local; the percentage of elongation reducing very rapidly as it is calculated in increasing lengths from the point of fracture. From the results reached as averages we submit the following statement of what we believe will be found to be average tensile and compression-tests of commercially pure aluminum, of composition varying in each constituent as follows: Aluminum, from 97 to 99 per cent. Silicon, graphitic, from 0.10 to 1.00 per cent. TABLE XVII. Average Tensile and Compression-Tests of Commercial Aluminum. Elastic limit per square inch under compression in cylinders, The modulus of elasticity of cast aluminum is about 11,000,000. 3,500 12,000 Taking the tensile strength of aluminum in relation to its weight, it is as strong as steel of 80,000 pounds per square inch. Comparative results calculated in this way are tabulated below, as taken from Richards's Aluminium. Mierzinski is quoted in Dingler's Magazine as giving the following results of tensile tests of aluminum wire: We have, however, obtained much higher results in tensile strength per square inch, and exhibit with this paper aluminum wire of No. 2 B. & S. gauge which pulled 49,500 pounds per square inch. Mr. Spilsbury, of the Trenton Iron Company, informs us that he has obtained, on aluminum wire of .061-inch diameter, 62,300 pounds per square inch tensile strength; the wire being drawn with one annealing from wire-rod of .160-inch diameter. The same wire made 47 bends to a right angle before breaking. Aluminum, when pure, is a very sonorous metal; bars of it suspended by fine wire, when struck, give a fine clear bell-like sound. The proper shapes for bells have not yet been devised. Bells of ordinary shapes do not give as good sound as do ingots of the metal. CONDUCTIVITY AND ELECTRO-POSITIVENESS. Mr. C. K. McGee, of the University of Michigan, has determined the electrical resistance of specimens of the "average lot 98.52 per cent. aluminum" metal, with the following results: In the electro-chemical series, aluminum is ordinarily placed near the positive end, being, under most circumstances, more positive than almost all the other metals, and only less positive than the metals of the alkalies and alkaline earths. That is, in most separations of aluminum by electrolysis it is charged with positive electricity, and separates at the negative pole. This arrangement is only approximate, however. Under some circumstances aluminum is electro-positive to sodium, as it reduces sodium when treated with its oxide or carbonate; again, it is electronegative to iron, as iron reduces aluminum from its sulphide. The attraction of aluminum for oxygen is exceeded by that of very few elements. If this attraction be measured by the amount of heat developed in the combination of aluminum and oxygen, it is about three times that of carbon for oxygen. That is to say, one equivalent of carbon uniting with two of oxygen gives 96,000 units, while two of aluminum with three of oxygen give 388,000 units of heat. In its behavior with most chemical reagents, at ordinary temperatures, aluminum resembles platinum and gold. It is like carbon and silicon, which are highly electro-positive at high temperatures, but electro-negative at low. Under some circumstances, as in nitric acid, aluminum is as electro-negative as platinum or carbon. It is suggested that at low temperatures the atoms of aluminum are combined with each other so as to render the metal inert. If its full chemical affinities were exhibited at low temperatures, it would be as easily oxidized by acid and water as metallic sodium. ACTION OF IMPURITIES. As observed under the head of Purity of Aluminum, silicon and iron form the bulk of the impurities; and these two elements will be treated first. Silicon hardens aluminum considerably, increasing its tensile strength without materially decreasing its ductility; it, however, very materially decreases its malleability, and takes away its capacity of taking a fine polish, and, much more, prevents its retaining whatever polish it has received. Silicon in aluminium oxidizes by action of the atmosphere or of moisture, and if present in proportions of over 3 per cent. very soon coats the metal with a dull gray and unsightly tarnish. For some purposes, where a harder surface is required than is given by pure aluminum, where advantage would be taken of the lightness of the metal, and where the surface can be lacquered or otherwise coated to protect it from oxidation, the alloy, say 6 to 8 per cent. of silicon, which can be readily made, can be used with advantage; but in all ordinary work to which aluminum is put, the more nearly free the metal from silicon the better. These remarks apply to the ordinary ways in which we find silicon, a large proportion of which exists in the graphitoidal form. Whether the influence of the amorphous silicon, could it be placed in the metal alone, free of graphitoidal silicon, would give the advantageous hardness without the tarnishing qualities, is an interesting question not yet answered. Iron in small percentages, as an impurity of aluminum, hardens it, but, of course, adds to its specific gravity and renders it magnetic. It also decreases the malleability of the metal, and, like silicon, detracts from the capacity of the metal to take a fine polish or to retain whatever polish it is at first susceptible of, although the alloy does not tarnish as rapidly as does the siliconized metal. For some purposes, where a harder and stouter alloy is wanted, a proportion of from 6 to 10 parts of iron works advantageously. W. J. Keep has pointed out the curious fact that the alloy of 50 per cent. iron and 50 per cent. aluminum, though melted together and seemingly forming a true alloy in the pot and in the metal as first cast, seems entirely to lose its power of cohesion and crumbles down in a little while to a powder, each grain of which seems to contain equal parts of aluminum and iron. Copper sometimes becomes an accidental impurity of commercially pure alumiuum, in proportions up to perhaps one-half of one per cent.; in such small percentages its influence is hardly noticeable in any of the properties of the metal, so far as has come to our notice. In larger proportions, up to 10 per cent., copper can readily and advantageously be alloyed with aluminum, especially for castings where hardness of surface is required. It takes away the peculiar polish of aluminum, and the fine gloss and peculiar color of cast aluminum; at the same time it adds hardness and decreases the shrinkage of castings, and for many purposes can be used advantageously. It has the disadvantage of adding materially to the specific gravity of the metal. Carbon only unites with aluminum under very high and continued heat, and then only in proportions not exceeding 3 per cent. Whenever it is found associated with aluminum the metal is brittle, porous and friable. Both Deville and Fremy say that, under ordinary temperatures, aluminum and sulphur do not unite or act on each other; indeed, Deville says aluminum may be heated in a glass tube to a red heat in vapor of sulphur without altering the metal. However, at very high temperatures, aluminum and sulphur do combine to form a sulphide, of the composition Al'S3. Ordinary aluminum of commerce is entirely free from sulphur. Lead is found as an accidental impurity of aluminum in proportions up to one-quarter of 1 per cent. In these small proportions, so far as yet determined, it has no appreciable action upon the properties of aluminum. In larger proportions, lead does not alloy with aluminum, and no homogeneous alloy, or even mixture of the two metals, can be easily obtained. Antimony does not unite with aluminum to form any homogeneous alloy. Chromium unites with aluminum readily, hardening it and adding to its tensile strength. An addition of 2 to 3 per cent. of chromium to aluminum improves it for many purposes, although, as in the case of all other impurities, it decreases the malleability of the aluminum. Tungsten unites with aluminum, hardening the metal, but not giving any very useful alloys. Platinum unites with aluminum readily; it, however, does not seem to give any advantageous alloys; they are brittle and unsound. |