more favorable conditions the dust would ignite directly from sparks, but it seems very improbable.
Let us continue now with the process through which the ground wheat is made to pass. From the stones it is conveyed to the bolting reels, where the very finest is sifted out first, and we obtain a grade of flour; after the finer material is sifted out it goes to a coarser bolt, where the "middlings," as it is called, passes through, leaving the bran which comes out at the end of the reel. The middlings, as it comes from the bolts, has fine bran and dust in it, and, to purify it, it is subjected to an operation similar to that of cleaning the wheat, that is, in the middlings purifiers it is subjected to a draught of air which takes away all the light bran and dust, leaving the heavier material (purified middlings), which goes again to the stones to be ground into flour.
Here is some of the dust from these "middlings-machines;" you observe it burns as the other materials burned, quickly, and with intense heat.
Here is some of the purified middlings; each grain is comparatively large and heavy, making it difficult to blow it well into the air, but, as the blaze produced by each particle is quite large, a flash is produced which does not differ materially from the others.
Here is some of the general dust of the mill, that is, dust swept up from the floors, walls, beams, etc. You will see it acts in all respects like the other substances.
And, finally, here is some of the flour taken this afternoon from the flour-sack at home; it burns, you observe, if possible with even more energy than the other kinds of dust.
I have performed a few experiments, which I will now repeat, which will illustrate to you the immense power that these materials exert when burned in a confined space.
This box (Fig. 2) has a capacity of two cubic feet; the cover has a strip three inches deep nailed around it, so that it telescopes into the box; there is in this lower corner an opening for the nozzle of the bellows, in this an opening for the tube to the lamp. I place now a little flour in the corner, light the lamp, and my assistant places the cover upon the box and steps upon it. Take notice that upon blowing through the hole, and filling the box with a cloud of flour, the cover comes up suddenly, man and all, until the hot gas gets a vent, and a stream of fire shoots out in all directions.
Here is a box (Fig. 3) of three cubic feet capacity, including this spout, nine inches square and fifteen inches long, coming from the top of it-at the ends doors are arranged closed like steam-boiler manholes; openings for light and bellows are arranged as in the previous box.
Here is a box, weighing six pounds, that will just slip over the spout; it has a rope lest it should strike the wall after the explosion.
