CH5(NO2), which is reduced to aniline, reduced to aniline, CH,(NH), used in aniline factories for the dyeing of silk, calico, and woollen fabrics. The action of HNO, and H.SO, on glycerine produces Nitro-Glycerine, C,H,(NO3)3, which when absorbed by kieselguhr is called Dynamite. Gun-Cotton, C12H,,O,(NO3) is produced by the action of strong nitric acid upon cellulose. Nitric acid is also used in the separation of gold and silver from their alloys, and for etching and engraving.

14

It

Nitrogen. N; density, 14.04. An element present in the atmosphere, of which it constitutes four-fifths of the total bulk; it is an essential constituent of animal and vegetable organisms, and of several chemical compounds. This gas, which is neither inflammable nor a supporter of combustion, is colourless, inodorous, and light, its specific gravity being 96. It liquefies at about 195° Cent., and solidifies at about 213° Cent. Nitrogen is an inert element, refusing to combine readily with other substances, and generally speaking its compounds are rather unstable. With oxygen it forms five oxides-nitrous oxide (NO), nitric oxide (NO), nitrous anhydride (NO), nitric peroxide (NO), nitric anhydride (NO); with hydrogen it forms ammonia (NH); with oxygen and hydrogen, nitric acid (HNO)..

In its liquid form nitrogen is colourless and mobile, and produces a greater lowering of temperature on evaporation than liquid air. When poured over a bulb of oxygen the gas becomes condensed on the inside of the glass in bluish drops. Poured over metallic calcium it forms calcium nitride, and on treating this with water, ammonia is given off.

Nitro-Glycerine. Also called propenyl nitrate, and nitro-glycerol, C,H,(NO3), is obtained by the action of nitric and sulphuric acids on glycerine. It is essential that both acids and glycerine should be quite pure. The mixed acids are placed in a leaden vessel, and the glycerine added slowly, the temperature being kept below 30° Cent. by cooling worms. A second apparatus separates the nitro-glycerine, which then goes through several washings and filtrations.

Nitro-glycerine is colourless when pure, odourless, possesses a sweet and burning taste, and

is poisonous. Sp. gr. 1-6 at 60° Fahr. It freezes at about 46° Fahr., but is not generally used either in the liquid or frozen state owing to the greater convenience of use when mixed with some absorbent material such as kieselguhr. In this form it is called Dynamite. With gun-cotton it forms blasting gelatin. Being unaffected by water, nitroglycerine is useful in sub-aqueous work, and was largely used at Hell Gate or Flood Island, in Long Island Sound, New York.

Nitro-glycerine requires to be fired by the sudden application of high temperature, and pressure, and so is always ignited by means of a detonator. This is a thin copper tube closed at one end and filled with fulminate of mercury and potassium chlorate, and the fuse is inserted through the open end of the tube in the fulminating mixture, being held in place by pinching together the open end of the tube. See Blasting.

Nonagon. A nonagon is a polygon with nine sides. nine sides. For its geometrical construction, calculation of area, &c., see Polygon.

Non-Conducting Coverings. The importance of these increases with continued increase in the pressure and temperature of steam employed. Coverings must not only be non-conductors, but also non-inflammable, and infusible, an important proviso. Hair felt, slag wool, and fossil meal are excellent nonconductors. In strictness no substance is an absolute non-conductor, but relatively some are five or six times more efficient than others.

Experiments have been made to determine the relative value of various substances. Those of Professor Ordway made for the Boston Manufacturers' Mutual Fire Insurance Co. are given in the table on the next page. This represents in a graphic manner the relative efficiency of materials which are wholly free from the danger of being carbonised, or ignited, by slow contact with pipes or boilers; (printed in ordinary type) and those which are more or less liable to be carbonised (printed in italics). Each of the non-conductors tested was used in a mass 1 in. thick, placed in a flat surface of iron kept heated by steam to 310° Fahr. The table gives the amount of heat transmitted per hour through each kind of non-conductor

classes of work and for timber constructions. It affords a good hold for the spanner, but cannot be turned so conveniently in a confined situation as the hexagon nut, B, because it requires a quarter of a rotation before the spanner can be put on again for a fresh grip, while the hexagon only requires a sixth. The proportions for hexagon nuts are given under Bolt; square nuts are the same size across the flats as hexagon ones of similar thread diameter. The convex faced nut c allows a little latitude of movement when parts do not align exactly. The face of the work is sometimes left flat, and a concave washer put under the nut. A flanged nut D is useful to cover up a large hole without the use of a separate washer; it is often employed in conjunction with a leather or other packing beneath to form a tight joint, especially

A

B

O

Normal Pitch.-The pitch of a screw gear or of helical teeth taken at right angles with the axis of the spiral.

Nut. Used in combination with a bolt or a stud, as a means of union. The square nut, Fig. 247, A, is used chiefly for the rougher

on pipes. The cap or box nut, E, goes a stage further, the end being closed in, to prevent the end of the screw from rusting, or leakage occurring past the screw. Wing or thumb nuts have a couple of projecting ears which are gripped by the fingers; a good many nuts on

machine tools, and other mechanisms are combined with short handles, to save the time of picking up a spanner, when frequent handling is necessary.

Circular nuts include those with flats F for a spanner, slots G for a hooked spanner, holes for a tommy, being set laterally as at H, or vertically, J, and slots K for a sort of bifurcated screw driver. The last two are only used when it is essential to sink the nuts flush with faces. See also Lock Nut.

Nut Making.-Nuts are produced either by forging, or by turning from bar. Black nuts are those left just as forged, faced nuts are turned on the faces, bright nuts are shaped or otherwise tooled on the body, in addition to facing, while for the best work hardening is done.

Forging is done from bar, the blanks being punched out, and subsequently trimmed or swaged. The tapping is done in machines of the type described under Bolt Screwing Machine, or more expeditiously in special

types with several spindles, fitted with carriers which take a string of nuts one after another over long taps. The facing operation is effected on a special type of lathe, provided simply with a headstock, and a rest; the nuts are run on to a short screw mandrel, which is fitted with a spherical washer, that adjusts itself automatically to the nut face, and allows the threads of the nut and mandrel to fit naturally, so that the resulting facing is true with the threads, irrespective of the previous condition of the rough faces. The tooling of the flats is done by shaping, milling, or grinding. When shaped, a large output is secured by mounting a string of nuts on a mandrel between index centres, which brings the flats round in correct positions; some machines have three or four such centres placed under a set of tools, so that several dozen nuts can be treated simultaneously. Milling is best done with straddle mills, embracing two sides at once. Since the advent of disc grinders, much finishing has been done on these, especially in the case of brass nuts.

Oak (Quercus).-This is the best known and most important of the hardwoods. It grows in nearly all parts of the world and is suitable for almost any purpose. Formerly in England it was by far the most commonly used wood, but circumstances have greatly reduced its importance. First, the home supply became inadequate, and imported oak was found to be of inferior quality. Then imported softwood became popular because it was light, cheap, and easy to work. New varieties of hardwood were imported, and some of them were found equal to or even superior to oak for special purposes. Finally iron and steel is displacing wood altogether for large structural work. Oak, however, must be regarded as the best all round wood that we have, and second to none for durability. Perhaps the most important work it is commonly used for now is railway wagon and carriage building. It is tough, fairly straight grained, flexible, greyish brown in colour, hard, heavy, and rather difficult to work. Boards cut radially from the trunk exhibit grain markings peculiar to oak, and the wood cut in this way is called wainscot oak.

Occlusion. Relates specifically to the absorption of gases by molten metals. This property they possess in common with fluids. This is the reason of blow-holes in castings. See also Fluid Compressed Steel. Hydrogen may be occluded by steel to the extent of five or six times its own volume. Nitrogen and CO are also occluded. Apart from the formation of blow-holes, there is no proof that occluded gases exercise any influence on the strength, or physical characteristics of steel. Small additions of ferro-silicon and aluminium prevent, or largely lessen the formation of blow-holes.

Octagon. A polygon with eight sides. For the mensuration and construction of the octagon, see Polygon. The following is a particular

Oddside Moulding. A method which economises the time of the moulder when a large number of similar moulds are required. It is adopted largely by brass founders, and in a less degree by iron moulders. To explain it briefly, the method of Turning Over may referred to. In this, using a two part box, the pattern is first bedded loosely in the box, which is subsequently to be the top, and the sand is only consolidated sufficiently to allow of a joint face being made on which to ram the bottom box. The temporary half mould, having served this purpose, is knocked out, after being turned over with the bottom, and is re-rammed properly on the bottom to form the real top of the mould. The making of this temporary joint has to be repeated for every mould, and when the joint is not plane it may often involve a good deal of work. Runners also have to be cut in one or both halves.