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Kindly keep your queries on separate sheets of paper when corresponding about such matters as patents subscriptions. books etc. This will greatly facilitate answering your questions, as in many cases they have to be reffered to experts. The full name and address should be given on every sheet. No attention will be paid to nnsigned queries. Fuji hints to correspondents are printed from time to time and will be mailed on request. (12572) S. B. asks: If a meter were attached to the wire or a five-horse motor, and tbe current (500 volts) turned on for, say, one hour, funning only single lllOtO!, would it use as mucb juice in one hour as if the motor were doing full five horses' work? A. If a lve borse-power electric motor is run without load, it will only require the current to turn it whieh is able to overcome the friction of the parts of tbe motor. If full load is put upon the motor, it will tben require five borse-power to turn it. This is exactly the same as any other machine, a grindstone, for instance. A boy can turn it swiftly when there is nothing grinding upon it; but if a man holds a scythe upon it, bearing down with all his force, even a man might not be able to keep the stone in motion. Power must be furnished to any machine in proportion to the work called for from the machine. (12573) G. W. M. asks: 1. What are the chemical elements found in the atmospbere? Is there any chemical change wben subjected to heat' A. Atmospheric air is made up of 21 per cent of oxygen, 78 per cent of nitrogen, about 1 per cent of argon, and a minute trace of belium, xenon, neon, krypton, hydrogen. No chemical change is produced by simply heating the air to any moderate degree, unless some other substance is present which can be burned with the oxygen. At the tempera lure of tbe electric arc the nitrogen and oxygen combine and form nitric and nitrous oxides. 2. How is commercial oxygen, used in medical science, produc(d? A. Oxygen for industrial and medical purposes is made from chloratc of potash or from binoxide of sodium. The' former is heated in a retort with manganese dioxide, and the latter is simply dropped into water. (12574) E. J. L. asks: WiII it be too much bother to advise me the numher of cubic feet in a perch of stone? Tbe di< tionary and printed authorities all seem to agree on 24%, but masons here claim that there are only 16% A. A “vereh” of masonry is not a lleasure whose value is everywhrre tbe same. It seems to be usually 1t '2 fcet long and 1 % by 1 foot, which would make it contain 24 % cubic feet. It is often tahrn at 25 cubic feet. b” * has other values in different places. If all such peculiar measures could he dropped and our work measured in feet. yards, etc., it would greatly simplify matters. (12575) J. B. C. asks how to oxidiz( silver. A. 1. Add four or five thousandths ammoniulIJ sulphide or potassium sulphide to water at a temperature of 160 to 180 degTees Fahrenheit. ''hen the articles are dipped into this solution an iridescent coating of silver sulphide is produced, which, after a few seconds, turns blue black if allowed to remain in the liquid. Remove, rinse, scratch-brnsh, and burnish when desired. 2. 'bere are two distinct shades in use, one produced by a chloride, which has a brownish tint. and the otber by sulphur, which bas a. bluish-black tint.. To produce the former it is only necessary to wash the article with a solution of sal ammoniac (ammoninm chloridf). 3. A much more beautiful tint may be obtained by employing a solution composed of equal parts of copper sulphate and ammonium chloride in vinegar (or dilute acetic acid). The fine black tint may be produced by a slightly warm solution of sodium or potassium sulphide. 4. Bromine, f gr.; potassium bromide, 5 dwt.; water, 10 oz.; boil the silver in this usually 2 to 5 minutes. then polish with rouge. 5. Dissolve sulphate of copper, 2 dwt.; nitrate of potash, 1 dwt.; ·ammonium chloride, 2 dwt., in a little acetic acid. Warm the article and apply the solution with a camel's hair pencil and expose to the fumes of sulphur in a closed box. Parts not to be colored must be coated witb wax. 6. Dip the clean silver article in a solution of sulphide of potassium (liver of sulphur), 2 dr. to 1 pt. of water. Heat this solution to a temperature of 175 degrees Fahrenheit. Immerse for a few seconds only, when the article becomes blue black. For a velvet black, dip the article, previous to oxidizing, in a solution of mercurons nitrate and water, and rinse. Then dip in tbe sulohide solution as above. For a brown shade, oxidize the potassinm sulpbide as above, then dip in a liquid composed of 10 parts of blue vitriol and 5 parts of sal ammoniac to 100 parts of vinegar. After oxidation, hrush with a scratch brnsh very lightly. to brighten and variegate the surface. 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