CORRESPONDENTS who expect to receive answers to their letters mnftt, iv all cases, sign their names. We have a right to knoio those who seek in formation from us; beside, as sometimes happens, we may prefer to address correspondents lyy mail. SPECIAL NOTE.—This column is designed for the general interest and instruction of our readers, not for gratuitous rep lies to Questions of a purely bunine$H or personal nature. We will publish such inquiries, however, ichenpaid for as advertisemets at $1 00 a line, under the head of Business and Personal. KWAll reference to back numbers should be by volume and page. W. W., of Ontario, Ca.—Vulcanized rubber may be soltcncri by heat, so as to become plastic. A. J. R., of Mich.—We cannot give time to the solution of j&ilch problems as you send us; you should apply to an hydraulic engineer. H. F. S., of La.—The use of a weak solution of ammonia in the water will be found useful in removing the odors caused by perspiration. M. L. B., 111.—We have not heard of any proposition made to Congress for.an appropriation to aid in the development of a system of telegraphing without wires. The best work on heat you can get is Prof. John Tyndalls. Heat considered as a Mode of Motion. D. B. K., of R. I.—We know of no work on rosette turning-, or the engraving engine. H. C. Baird, 406 Walnut st. Philadelphia, Pa., or D. Van Nostrand, corner of John st. and Broadway, New York, may accommodate you. F. H. B., of--------answers the question of T. K., of Mich. on page 124, current volume, as to how many pounds will be required to depress eight springs etc. by one pound only. A. P. W., of 111., asks the rule for determining the relative resistance of a steelplate of a given thickness and one composed of many layers but of the same aggregate thickness. As the word resistance la of a general and not a specific significance, and he does not say whether the resistance is against pressure, percussion, torsion, extension, or otherwise, we cannot reply to his question. Why cannot inquirers write what they mean ? C. 0. H., of Iowa.— What number of revolutions should I run a 24-inch circular saw cutting oak into stove wood? 9,000 feet per minute is a good general rule for the speed of the periphery of circular saws. At this rate your case would stand thus in round numbers : 9,000 divided by 6 feet, circumference of saw, equals 1,500, number of revolutions of saw per minute. H. M. C, of Wis.—Copper and brass may be coated with zinc by boiling them in a solution of sal-ammoniac with zinc turnings. J. R., of Mass.—Cotton may be detected in linen as follows : Immerse a piece of the suspected tissue in boiling solution of hydrate of potash and water contained in a glass or porcelain capsule, equal parts by weight, for three minutes, then remove it and dry between blotting paper; the cotton threads will show either white or very light yellow, while the linen fibers will be a dark yellow; a small lens will aid in the examination. B. M. R., of Va.—Plumbago or black lead as it is sometimes called is a compound of iron and carbon, not an oxide of iron as you suppose It is the best material for lessening friction between wooden surfaces where oil is not admissible, and is generally used by piano makers and tuners forthat purpose. B, M. GK, of Mo.—Portable glue is made by adding one part of brown sugar, by weight, to two parts of good glue in solution straining and cutting into pieces while yet soft. It is much more soluble than the common glue, and is useful for many purpose?, as fastening paper to drawing boards, etc R. T. McM., of Del.—Mock ivory may be made by mixing isinglass and strong brandy into a paste with finely powdered eggshells. It should be cast in molds previously well oiled, and be left to dry. It is said to resemble real ivory when hard. E. E., of Indiana.—Dross is the oxide which forms upon the surface of metals when melting, often mixed with other impurities. Zinc when deposited on the surface of iron in the process of galvanizing assumes the crystalline appearance of which you speak. Sal ammoniac is the name of the salt, used by tinsmiths in soldering and in tinning iron. C. F. B., of K Y.—The strength of malleable cast iron and wrought iron, vary with the perfection of the methods employed in their manufacture, and the quality of the iron. Malleable cast iron often approximates closely in strength to wrought iron. Its strength also varies with the size of the casting. As a rule small sized castings have the greatest tensile strength. You will see therefore that an exact ratio ol strength between these materials cannot b e established. J. P. McG., of Tenn.—The complete deodorization of coal oil, has not yet boen fully accomplished. Coal oil will mix with certain saline solutions, and some other hydro-carbons. The solvents of sulphate of quinia are numerous. Cold water dissolves it sparingly. Thirty partn of boiling water dissolve it entirely. Sixty parts of cold alcohol dissolve it completely. Ether dissolves it with difficulty. All, or nearly all, the dilnte acids dissolve it with great facility. We believe it is also soluble to some extent in glycerin. A. S., of Conn.—Shellac is dissolved by alcohol, muriatic, and and acetic acids. Solution of potash also dissolves it and combines with it chemically. The insect commonly called the deathwatch is a species of beetle, of the genus anobium. It is the larvae of this insect that causes the sound resembling the tick of a watch; it is the sound of their mandibles as they gnaw upon the wood in which they are concealed. R. 0. S., of Ky.—A good black ink is made by the following recipe. Bruised Aleppo nutgallslS lbs. boiled an hourih copper, with six gallons of water, adding enough to compensate for evaporation. Strain and boil one half hour the same galls with four more gallons of water. StrainandboilthegallsagainwithS gallons of water; strain again and mix the several liquors. Now heat and add while hot 4 lbs. of coarsely pulverized copperas, 3 lbs. of bruised gum arabac, and stir until all ia dissolved. Let it settle and finally strain through a hair sieve. The addition of a little creasote or carbolic acid will prevent mold. The addition of 1 lb. of sugar will make this a good copying ink. T. J. L., of Pa., referring to an article in a previous number of the SCIENTIFIC AMERICAN, on Steam Igniting Combustible Substances, asks il the action of the steam is not purely mechanical, as when a jet under high pressure impinges on some easily ignitalDle substance, thus producing friction, consequently heat. In reply we ask would a jet of atmospheric air under similar circumstances produce ignition? The mechanical conditions are the same, but the heat is wanting. P. S. B., of Mass., asks for iron cements. We understand he desires the recipes for making cements for uniting iron, as pipes, etc. The best quick setting cement we know,may be made by mixing powdered sal ammoniac, 1 part; powdered sulphur, 2 parts; iron filings SO parts, all by weight, with sufficient water to make a paste of the required consistency. The red lead cement for face joints is made of white and red lead, equal parts, mixed with raw linseed oil to the proper consistency. J. P. J., of R. I.—Riveted splices of belts are much to be preferred to those secured by lacings. In the latter case the belt is more or less injured by the use of the belt awl. Always use a punch in preference to tlie barbarous awl.