CORRESPONDENTS who expect to receive answers to their letters must, in all cases, sign their names. We have a right to know those who seek information from us; beside, as sometimes happens, we may prefer to address correspondents by mail, SPECIAL NOTE.—This column is designed forthe generalInterest and in struction of our readers, not for gratuitous replies to questions of a purely business or personal nature. We will publish such inquiries, however, whenpaidfor as advertisemets at $i'00 a line, under the head of "Business and Personal." TWAll reference to back numbers should be by volume and page.. G. W. K., of D. C.—We have seen tolerably good specimens of American Eussia-sheet iron, but nothing equal to the imported. C. A. S., of Gasoline is so exceedingly volatile that its evaporation can be prevented only by keeping it in hermetically sealed vessels, of non-porous material. You will find answers to your other inquiries in any elenentary text-book on chemistry. J. T., of N. Y.—No substance known can be positively asserted to be a simple substance or element. The possibility of discovering elements in the baser metals, which will unite to form the precious metals, of course implies the recombination of those elements to form the baser/metals. E. M. S., of La.—A splendid blue writing fluid can lie made as follows: Take pure Prussian blue six parts, and oxalic acid one part, . mix with a little water and rub it into a perfectly smooth paste. Then dilute with rain water to the proper consistency, and add a little gum-arabic to prevent the spreading of the ink. R. R., of Ohio, writes us that in the discussion relative to the floating of solid on melted iron, the fact that white or chilled iron will sink and gray iron will float has not been mentioned. Reference to this statement may serve to throw some light upon the discrepancies in experiments as hitherto recorded. We would inform this correspondent in reply to his inquiry that, red hot iron has as high a temperature as the flame generated in the combustion of many substances. H. and Co., of W. Va.—The "proper speed of a mulay saw to cut the most lumber " depends on the quality of that lumber. It will vary according to this. circumstance from 200 to 800 revolutions, or double strokes perminutV The" proper spe'ed of a circular saw is 9,000 feet per minute for the edge; thus in case of your 54-inch circular saw it would be: j 14 feet, the circumference, 9,000 feet, the speed.product by division 648, the I number of revolutions. If the. lumber is soft wood and clear 700, or even 72J revolutions may be advantageously used. J. H., of N. J., can bronze his gun barrel by diluting either nitric or sulphuric acid with its volume of water and applying it to the . barrel with a rag. Be sure the barrel is perfectly clean. This cleanliness can be assured by washing the barrel with lye or soap suds and rubbing dry with cocoanut husk. Several applications of the acid may be required, but one is usually enough. When the tint is obtained wipe off with an oily rag. TJ. E., of N. J.—We do not approve of leading the exhaust steam into a brick chimney stack, as it tends to disintegrate the mortar It will, however, increase the draft. Better build the chimney higher. B. H., of Mich.—We have already given detailed descriptions, generally illustrated, of all the notable improved firearms in this country and Europe. They are to be found in back volumes from XIV. up. The galvanic or electro-magnetic battery is fully described in almost any work on chemistry or natural philosophy. W. W. T., of R. I, says he has a gear of 100 teeth, pitch 18 to the inch, what thread shall he cut on a worm to drive it ? If the gear teeth are 18 to the inch, of course the worm must be the same pitch—18. one revolution of the worm moves the gear the space of one tooth. J. N, H., of Canada, asks where the best smoke consuming apparatus, the best paint and putty mill, and the fixtures for using liquid fuel may be obtained. D. W. H., of Iowa.—Your explanation of the inside and out-side crank pins in reply to the inquiry on page 151, current volume, Scientific American, is correct, but altogether unnecessary. A. B., of Tenn.—We cannot understand how you can use the condensed steam for a blast or draft after heating your feed water with it. Condensed steam is water. The capacity of a boiler is increased by heating the feed water—we mean the capacity for producing a given amount of steam in a given time. A pipe one-and-a-fourth inches diameter is sufficient to supply a steam cylinder 8x18 inches unless the pipe is very long, crooked, and unf elted. J. W. H., of Minn., asks if a belt [running at a speed of 2,400 feet per minute will transmit more power than the same b elt running 1,600 feet per minute. Of course it must; it requires more power to drive it at the greater velocity and that power is not thrown away. Velocity is one of the manifestations, if not an element, of power. C. H. P., of 111.—We have lately published recipes on cements and mucilages. The bases of them are starch, gum-arabic,'dextrine, or gum tragacanth, dissolved in water and preserved by a small addition of alcohol or acid. B. E. P., of N. Y.—The occurrence of a partial or complete explosion in a kerosene lamp upon the slight turning down of the wick, may be accounted for by supposing the heat to have generated gas In the lamp, which could not readily escape, until the turning of the screw opened some small aperture. This viewis sustained by the sound you describe as of escaping steam. If the wick was drawn in tight, when saturated with oil it would prevent the escape of. the gas, until lowered. The orifices by the side of the burners you describe might easily become stopped by concreted oil. The best kerosene oil will be converted into gas by heat. J. B., of Pa.—This correspondent asks how many horsepowers are required to drive an eight or ten inch circular saw, running entirely in wood. He says he runs an eight inch saw through one inchboard, turning with one hand. The question is indefinite. The speed of the saw, its thickness, whether ripping or cross-cut, the sort of wood sawed, etc., shonld be known before a definite reply could be made. M. E. H., of Iowa, says he has laid 4,000 feet of two-inch pipe from'a spring which is 30 feet higher than the delivery end, but the water rises at that point only 15 feet. The pipe runs in a Btraight line, having a descent of 18 feet the first 1,000, the remainder level to the upright delivery. In this case there can be no reason why the water will not rise to the level of the head, less the friction, which, however could not retard the water to the amount of 15 feeti The pipe has a leak somewhere in its length. H. M. S., of Ohio.—We do not remember one instance in which Congress has ever been asked to repeal a patent. It is not likely that any such application would be acted upon, unless very special reas ons could be shown.