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Upon blowing the dust into the air, the explosion following raised the cover enough for a sheet of fire to stream out several feet in all directions.

A very large and heavy packing box was then taken, arranged with a safety valve to get if possible some quantitative results. Two holes with valves opening inward were arranged for two pairs of bellows. It was discarded, however, for two reasons; first,

Chem-Eng.

Fig. 3.

it was found to be very hard to produce sufficient dust without blowing the light out; and, second, when it did produce an explosion, the sides of the box having so much surface, would bulge, suggesting an explosion of the whole apparatus. Also it would keep the cracks where the box was fastened together. Another smaller box was nade with this arrangement to ascertain the amount of pressure produced by a given amount of dust; but the amount of dust blown into the air was different each time; the purity of the air was not constant; all the dust thrown into the air would not always burn, and some would be blown out of the hole used for the bellows.

The averages of the pressure upon the valve taken in this way was eighty pounds to the square foot, when one ounce of dust was consumed in the box.

No corrections were made, however, for the loss of dust as above. This result is smaller

than would be obtained under favorable circumstances. The safetyvalve was then removed, and a box 10 x 9 x 16 inches, bound around the hole. with iron wire, and weighing six pounds, was placed over When from two to two and a half ounces of dust was exploded in the larger box, the hot gases rushing out of the hole would project the smaller one from six to twenty feet into the air, the height depending upon the amount of dust consumed and its conditions as to dryness. (Fig. 3).

It makes an interesting experiment to fasten a thick paper over the

opening and explode some dust within. The paper bursts with a loud report and takes fire. (Fig. 4).

In all cases the force of the explosion seems to be greater if vent holes are made around in the box, through which the air can enter, but the force of the explosion under these circumstances, of course, could not be measured as it would be expended largely through these openings.

Experiments made with dust from a furniture factory gave all of the above results.

The state of the atmosphere has a great deal to do with the success of the above experiments. All sorts of flour dust absorb moisture very rapidly. Upon exposing some in a pan over night in a damp room it was found impossible to make it explode in the box, but in the open air over the gas jet it would still burn.

After several explosions in the above boxes, in rapid succession, the dust became very hot. In this condition particles formed into little loose bundles about the size of a pen holder, which not only smouldered, but actually blazed enough to set the sides of the box on fire. Experiments show that the dust from middlings machines is no more explosive than the dust from stones grinding wheat, hence the theory that the new process of manufacturing with middlings machines is any more dangerous on any such account is. I think, erroneous.

It is a draught of air carrying a thick cloud of dust that is dangerous; such a cloud if ignited in any way explodes, and if large enough, throws flour, middlings, etc., into the air, when the explosion becomes general.

Of course, if machines are used which throw dust all over the mill, and it is not cleaned up often, it makes a general explosion, more severe; but again, if the machine is coveerd, and the dust conveyed in dense clouds. through spouts and dust rooms, the chances for producing explosions are increased.

From the above experiments and the testimony before the jury, the following suggestions may be worthy of the consideration of millers and mill builders:

First-Never use an open light in any part of the mill.

Second-Never light a lantern or strike a match for any purpose in any part of the mill where there is any dust at all.

Third-Never use an ordinary lantern in a spout or bin where there is any draught of air carrying dust with it. One end of such spout or bin should be closed before the lantern is introduced. A lantern should not be placed in such a locality if it can possibly be avoided, and if absolutely necessary a light covered with very fine wire gauze would be much safer.

Fourth-Avoid as much as possible in the construction of mills the conveying of materials by fans.

Fifth-The flour and middlings elevators and spouts should be constructed with a view to prevent air draughts through them.

Sixth-Dust houses, especially the flour dust house, should be constructed of sheet iron on all but one side; that side should be of light material and be exposed to the open air outside the mill, that is, it should form a part of the side of the mill.

Seventh-The dust spouts, especially those from all stones, should be of sheet iron, while the end away from the dust house should pass through the wall of the mill and be covered with light material.

Under these conditions there would be less danger of fire, as there would be no wood to ignite from glowing or charred masses of dust, and in case the dust should explode, it would not be communicated to the mill but blown out the end of the spout in one direction, and the side of the dust house in the other.

The dust house should be as small as practicable. The objection to the above will be made that the steam and dust will make a mush in the spout and house, on account of the rapid radiation from the iron. In the dust

Chem Eng.

Fig. 4.

house the sides could be covered with wood upon the inside. If now the fan is placed beyond the dust house, and the air is drawn through the eye of the stone, no steam would condense until reaching the dust house.

By actual test it is found that the materials coming from the stones have a temperature ranging from 120 degrees to 140 degrees, and the air at this temperature would carry all the steam from wheat without condensation.

It is admitting cold air to the spout after the dust has left the stones that produces the wet dust in the spout. These iron spouts might be covered with wood to prevent radiation.

In case the above was found not to be practical, the spouts and houses as now constructed could be covered with heavy sheet-iron, so that they would resist the explosion enough to have all the fire blow out of the mill.

THEORY OF THE EXPLOSION AS DEDUCED FROM THE TESTIMONY.

The evidence shows conclusively that the explosion commenecd in the Washburn mill. It is proved by the witness Lund, that there was a smouldering fire in the dust house, or the spouts leading to the dust house, in all probability the latter. There is but one way in which it is barely possible

for a fire to originate in such a locality, and that is by sparks from the stones igniting little masses of loose dust, (as in the experiment before mentioned), the latter being fanned into a blaze by the air draft. This blaze probably ignited all the dust in the spout and flour-dust house, which increased the pressure enough to burst the house and knock out the end of the spout, producing the "handful of fire" seen by Mrs. Stevenson, coming from the corner of the mill. This was followed by the explosion of all the dust in the grinding-room, which had been thrown into the air by the bursting of the flour-dust house, producing the first of the minor flashes seen by witness Stevenson. Then, as stated by this witness, the fire shot out of the lower windows on the corner and was sucked in again, (that is, probably the wheat dust forced out, flashed and went out, looking from his position as if it returned) then jumping from floor to floor, producing a comparatively clear but faint report in each story.

Light travels at the rate of 190,000 miles per second, and sound 1,120 feet per second, which accounts for Stevenson's statement that there was no report to the first flash; he saw the second flash before hearing the first report.

To witnesses still nearer, in front of the Empire mill, the rushing of the hot gases out of the doors and windows produced a noise that so drowned the separate reports from floor to floor that the sound of "z-z-z-zip" was produced; a combination of little independent reports and a constant rushing noise; witnesses still nearer, as McDaniel, in the "B" mill, heard no clear report, but were affected by a violent shock, which threw them down. Witness Day, nearer than any one else, heard no report at all, but formed, in fact, a part of the great sound wave. In an organ pipe the air moves up and down upon the inside in the direction of its length. The motion sends a vibration through the air around the pipe, which, striking the ear, produces the sensation of sound. Imagine now an immense organ pipe with a man in it, moving up and down with the air; he would hear nothing. So it was with Day. The sound wave was so strong where he was that it did not move the drum of the ear only allowing the body to remain stationary, but took the body and all across the room and back again.

The outward impulse was produced by the expansion of the hot gases formed first by loose dust in the mill, but as these rose into the air, tending to form a vacuum below, the air from all directions rushed in, acting upon the flour, middlings, etc., that was in bulk, as the bellows in the experiments given, throwing it in all directions and adding to the power of the explosion. Probably, when the roof was a hundred feet high and the walls in all directions ten feet, the force of the explosion was much greater than at first. The flour dust, middlings, etc., produce for a comparatively long time. for an explosive, a uniform or slightly increasing force, which would give to a projectile uniformly accelerated motion. It would not burn like the fine gun powder, but very much slower than very coarse powder.

Although separate minor explosions are spoken of as taking place in

each story, they were not loud enough to be heard by persons near the disaster, nor at a considerable distance. They were, in fact, but parts of the one first, great explosion. A rush of air sufficient to carry a man from one side of the room to the other and back again, in an adjoining mill, would throw all the dust in the Diamond and Humboldt mills, where the windows were open, into the air, the flames from the Washburn would rush in and ignite it, and explosions two and three would ensue.

It can be seen by inspecting the ruins that the walls of the Washburn mill fell first; the stones from the walls of the Diamond upon the back side lie in regular tiers upon the Wall of the Washburn, and the stones from the walls of the Humboldt lie upon the walls of the Diamond over the ruins of Butler's shop. The foundation walls of all the mills lean outward upon every side. All of which goes to show beyond a doubt that all three mills exploded, and also that they exploded at different times and in the orderWashburn, Diamond and Humboldt.

A piece of sheet iron from the elevator, two feet by eight feet in size, was picked up below Bridal Veil falls, two miles from the disaster, which fact, together with the evidence as to the appearance of the top of the elevator immediately after the explosion, makes it seem possible that the dust in this building formed a part of the explosion.

(To be continued.)

L. W. PECK.

During the last few years there has been a marked increase in the imports of nickel in ore and matte into the United States. In 1906 these imports were 15,156 tons, containing approximately 18,500,000 lbs. of metallic nickel, having an invoice value of $1,816,631, and showing a substantial increase over previous years. The import duty on nickel, in pigs, ingots, bars or sheets, or as an alloy, is 6 cents per lb. ; but ores and matte are free. The domestic exports last year consisted of 10,620,410 lbs. of nickel oxide, and matte, valued at $3.493,643, which is considerably more than was reported in either 1905 or 1904.

The production of nickel from domestic ores is small; hence the United States depends for its supplies chiefly upon Canada and New Caledonia.

The Wagner Water Still Co. has moved from its old offices at 89 Washington St., Chicago, to new and larger offices in the Northwestern University Bldg., at the corner of Lake and Dearborn Sts.

The company is prepared to handle promptly all orders for the New Wagner Twentieth Century Automatic Water Still, and a full set of circular matter will be furnished to any one interested.

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