arisen from a reckless use of the material. The endeavors of Mr. Noble to diminish the chances of the accidental explosion of the liquid substance led to the production by him, in 1867, of the solid preparation of nitro-glycerine, known under the name of dynamite, which constitutes, as now manufactured, one of the safest, most powerful, and most convenient explosive agents. Dynamite, as originally prepared, consisted of about 75 per cent. of nitro-glycerine absorbed by about 25 per cent. of a porous, infusorial, silicious earth found in Germany. This substance is now furnished to the trade in cartridges whose charges have the consistency of dry putty. During the late siege of Paris attempts were made in that city to substitute some other earth for that used in Germany. The most efficient absorbents were found to be silica, tripoli, alumina, and sugar. None of these, however, is considered to be so efficient as the German mineral. The preparation called glypto-fractur is stated to be a secret composition, but it has been publicly acknowledged by the manufacturers that the material is a modified dynamite, for which, however, special merits in regard to safety and pow er are claimed. It is difficult, however, to conceive that it can be more powerful than the original pure dynamite, from which it differs principally in containing a larger percentage of earths. Professor Abel believes that Noble's dynamite has a decided superiority over glypto-fractur in respect to the retention of nitro-glycerine at different temperatures. Exploding nitro-glycerine in its pure or liquid form is preeminently dangerous, and therefore only likely to receive exceptional application.

Professor Abel makes a comparison between the destructive effects of dynamite and compressed gun-cotton, and states that, generally, in all operations where rapid destruction is to be accomplished, gunpowder is undoubtedly inferior to these new explosive agents. Not only would a much larger quantity of powder be required to produce similar results, but in some cases it would be impossible to perform the same operations. In tunneling in the slate quarries of North Wales, a work that costs 60 shillings per cubic yard for gunpowder is done in less time for 45 shillings per yard with compressed gun-cotton. In experiments made by Mr. Hawkshaw, it was found that a detonation of gun-cotton

charges, placed simply upon the surface of submerged soft chalk rock, would break up the latter so as to facilitate its rapid removal by dredging, the rock being completely disintegrated, or pounded into a plastic mass like clay. Comparing the effects of dynamite containing 75 per cent. of nitro-glycerine with those of gun-cotton, the two materials appear to be practically on an equality, weight for weight, but the results furnished by either of them are accepted as being about six times those produced by gunpowder. The most prominent advantage of dynamite over gun-cotton is that it may be used in a damp hole without fear of its missing fire; while, on the other hand, compressed gun-cotton possesses the advantage that it is not in any way injurious to handle, is not at all affected in its explosiveness by cold, and may be preserved for any length of time without deterioration in its damp and unignitable state. The vapors evolved from the explosion of either of these are decidedly more objectionable than gunpowder smoke.-Abel on Explosive Agents.

AN INGENIOUS AND NEW MOTOR.

According to the laws of the mechanical theory of heat, mechanical work can be produced by the employment of any differences whatever of heat; and a very simple device has been invented by Bernardi, which is curious, if not of industrial value. Two glass globes are united by thin metallic arms to a central drum, the arms being bent at right angles when they enter the globes. The globes are partly filled with ether. An axle passes through the central drum in such a way that as it revolves, carrying the arm and the globes with it, the globes successively dip into a basin of cold water. Each globe is covered by a very fine network, which, becoming wet in that part of its revolution that carries it below, is subsequently, during the greater part of its revolution, exposed to the air. Evaporation of the water over the exterior of the network, and the consequent cooling of the globe, causes a slight condensation of ether in its interior, more of which is supplied from the opposite globe then being immersed in water, the upper or exposed globe becomes the heavier, and by its tendency to sink keeps the axle in continued but slow rotation, which does not cease so long as

water is supplied to wet the globes. A number of such pairs of globes being fastened to the axle, sufficient force is produced to turn a delicate clock-work. An apparatus constructed by Bernardi, with globes having a diameter of three fourths of an inch, and with arms three inches long, has worked for three months without change; in which period he calculates that the quantity of heat consumed by the apparatus has been equivalent to sixty revolutions of the wheel per day.-13 B, III., 80.

THE PYROLETER.

A new apparatus for the preservation of life and property from fire at sea, called the Pyroleter, or fire destroyer, was lately tested in England. A barge of some 40 to 50 tons' burden was fitted up for the purpose of the experiment. Along the entire length and width of the hold cotton-waste, shavings, and small wood saturated with oil and naphtha was placed to about the depth of two feet, and ignited on a given signal, within two minutes of which time dense volumes of flame and smoke issued from the open hatchways. The hatchways were then battened down, and the apparatus being set to work, the flames were completely extinguished within four minutes. The Pyroleter, by means of which this result was effected, is a small pump, which draws from tubs placed on each side of it simultaneous supplies of diluted hydrochloric acid and a solution of sodium bicarbonate. Both mixtures then meet in a generator and instantaneously pass into a separator, whence dry carbonic acid gas is evolved, and passes through fixed pipes to the locale of the fire, which it speedily suppresses. The chief merit of the invention is that a fire can be readily extinguished by dry gas with the assurance of no damage to the cargo therefrom.

ETCHING IRON.

Much time and attention has been devoted by Professor Kick, of Prague, to the subject of etching iron with acids. His method for arriving at a knowledge of the quality of iron or steel is not a new one, having been used with some success for a long time, but the care with which the Professor has conducted his experiments makes them exceedingly valuable.

Some kinds of iron exhibit what is known as the passive state, and are unacted upon by acids until this state has been destroyed by heating. The surfaces thus prepared are inclined to rust very soon. After a series of experiments with nitric, sulphuric, and hydrochloric acids, and etching solutions of copper salts, Professor Kick found that a mixture of equal parts of hydrochloric acid and water, to which was added a trace of chloride of antimony was the best etching solution. The chloride of antimony seems to render the iron less inclined to rust, so that after washing thoroughly in warm water, and applying a coat of Damar varnish, the etched surface may be preserved quite clean. The smooth surface that is to be etched is surrounded by a ridge of wax an inch high, as is done in etching copper for plates, and the acid is poured into the disk thus formed. At a temperature of 55° to 65° Fahr. the action soon begins, as shown by the gas evolved; in winter the etching is poor. The time required is from one to two hours, but the etching should go on until the texture is visible. Every half-hour the acid can be poured off without renewing the wax, the carbon rinsed off, and the surface examined. If too much chloride of antimony is added to the acid, a black precipitate will soon form, which can easily be distinguished from the carbon. drop of chloride of antimony to the quart of acid is sufficient. When the etching is finished the wax rim is removed, the iron washed first in water containing a little alkali, then in clean water, brushed, dried, and varnished. If in a few hours it begins to rust, the varnish should be removed with turpentine, which will also take off the rust, and then varnish again.

The appearance of different kinds of iron, when etched, is essentially as follows: Soft or sinewy wrought iron of excellent quality is attacked so equally by the acid, and so little acid is separated, even after several hours' action, that the surface remains bright and smooth. Fine-grained iron acts the same; the surface is still smoother, but a little darker. Coarse-grained and cold-short iron is attacked much more violently by acid than that just mentioned. In ten minutes, especially with cold-short iron, the surface is black. After thirty minutes a black glass can be washed off, and the surface. will remain black in spite of repeated washings, and exhibits

numerous little moles. Certain parts of the iron are usually eaten deeper, while others, although black and porous, offer more resistance. By allowing the acid to act for an hour or so, then washing, drying, and polishing with a file, a distinct picture is obtained. Malleable cast iron, we know, rusts more easily than wrought iron, and it is an interesting fact that the action of acids is also violent, the surface being attacked very intensely. Gray pig-iron acts like steel; the etched surfaces have quite a uniform gray color. In puddled steel the color of the etching and washing is gray, with a uniform shade, and the lines are scarcely visible. Cement steel has a very similar appearance, the lines being very weak. In Bes semer and cast steel the surfaces etched are of a perfectly uniform gray color, with few, if any, uneven places. The softer the steel the lighter the color. On etching, the finest hair-like fractures are rendered prominent. A piece of steel, which looked perfect before etching, afterward exhibited a hair-like fracture throughout its whole length. When different kinds of iron are mixed, the acid attacks that for which it has the greater affinity, while the other is less acted upon than if it were alone. Etching is exceedingly valuable to all who deal largely in iron, as it enables them to determine with comparative accuracy the method of preparing the iron, as in the case of rails, etc., as well as the kinds employed. -3 A, October 4, 1874, 523.

SCHMITZ'S REVOLVING FURNACE-BARS.

The London Iron speaks with much approbation of Schmitz's Revolving Furnace-bars, in which the ordinary straight fire bars are replaced, singly or in pairs, by hollow cylindrical bars, pierced with holes, and so arranged as to be easily capable of revolution. These bars rest on supports which are themselves cylindrical and hollow, and are sup ported lengthwise by a plate beneath the door of the firebox, and fitting into a neck made at the near end of the bar. For revolving them a winch is inserted in the hexagonal opening in the front end of the bars, by which they are turned.

The lighting of the furnace is performed in the ordinary way, and the furnace door can be kept completely closed, the perforations of the hollow bars supplying as much air as