non confined to a thin shell of the sun's outer envelopes; and compounds formed would be rapidly decomposed both by dissociation and chemical reduction by being swept down into the underlying hotter regions by the convection currents which take place on such an enormous scale in the sun's atmosphere. The heat of the zone of combustion may also contribute to the dissociation of compounds formed therein.*

It is well known to spectroscopists that the solar spectrum is never absolutely free from the so-called "telluric" lines, which have been shown to owe their existence to the aqueous vapor of our atmosphere. It is possible from the present point of view that these lines may be partly caused by aqueous vapor in the higher regions of the sun's atmosphere. Should there be any connection between the activity of combustion and the formation of sun spots, a rigorous comparison of the "telluric" lines in the solar spectrum carefully observed (or still better, photographed) at different periods of the spot cycle would be of the highest possible interest. Thus it may be suggested that the solar combustion varies periodically in activity-combination being in excess of dissociation during one half of the cycle, and dissociation being in the excess during the other half, when the heat resulting from the combustion, having reached its maximum, tends to decompose the compounds formed. This view points to the belief that the connection between the sun spot period and the period of variation of magnetic declination may be due to a common cause-the activity of combustion in the sun's atmosphere and the resulting variation either in the amount of free oxygen, or in the magnetic characters of this gas consequent on variation of temperature.

Sir William Thomson's theory of the dissipation of energy leads to the belief that the sun, like other stars, is gradually cooling down. Thus we should be led to infer à priori that there must be a period in the life of a star when compounds can begin to form. Such combination would begin in the outer and cooler portions of the star's atmosphere, as required by the present hypothesis, and would be attended with the development of the heat representing the energy of chemical separation. As the star goes on cooling down, the zone of combustion, at first a mere shell, would gradually encroach upon the central regions, and a star having permanently bright *See Bunsen's experiments on the combustion of different mixtures of CO and H with O (Pogg. Ann., cxxxi, 161); also Berthelot On the Chemical Equilibrium of C, H and O (Bull. Soc. Chim., [2] xiii, 99).

I may here recall the much discussed observation of Secchi, who asserted the existence of water vapor in the neighborhood of sun spots (Compt. Rend., lxviii, 238). Janssen also, in 1864, observed aqueous vapor in the atmosphere of Antares, and, in 1868, in the atmosphere of many other stars (Compt. Rend., lxviii, p. 1845).

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lines in its spectrum would result. In the earlier stages of what may be called the "chemical period" of a star's historya period into which our sun may be supposed to have entered -the lines of the non-metallic elements would alone appear bright, for the reasons detailed in the foregoing portions of this paper (paragraph 11), and, owing to their comparative faintness, would be lost at the enormous distances which the light of the star has to traverse before reaching our spectroscopes. When, however, the region of combustion had encroached sufficiently to reverse the metallic lines, these would shine out with much greater brilliancy than the non-metallic lines, and we should have a background of continuous spectrum crossed by the bright lines of the metals of smallest vapor density. Such stars would only be expected among those which are, so to speak, in the latest phase of their "chemical period." It is significant that 7 Cassiopeiæ, & Lyra and Argo, three stars which show bright lines in their spectra, all have sufficiently complex spectra to warrant the belief that they have entered upon a late phase of their existence. Before the actual reversal of the metallic lines there must exist a period in the life history of many stars when the temperature and extent of the zone of combustion is such as to obliterate the dark lines of those metals which will ultimately appear as bright lines. Such appears to be the case with the hydrogen in a Orionis; and according to the present views it might perhaps be predicted that this star will sooner or later show a permanent hydrogen spectrum of bright lines. It is conceivable that in certain cases the composition of a star's atmosphere may be such as to permit a considerable amount of cooling before any combination took place among its constituents; under such circumstances a sudden catastrophe might mark the period of combination, and a star of feeble light would blaze forth suddenly, as occurred in 1866 to Corona Borealis. In other cases, again, it is possible that the composition of a star's atmosphere may be of such a nature as to lead to a state of periodically unstable chemical equilibrium; that is to say, during a certain period combination may be going on with the accompanying evolution of heat, till at length dissociation again begins to take place. In this manner the phenomena of many variable stars may perhaps be accounted for. On the whole, the possibility of actual combustion taking place in the atmosphere of a slowly cooling star previously at a temperature of dissociation does not seem to me to have had sufficient weight attached to it; and in concluding, I would point out the important factor which is thus introduced into calculations bearing upon the age of the sun's heat in relation to evolution.

London, June 6, 1878.

ART. XXXIV.-On the Explosion of the Flouring Mills at Minneapolis, Minnesota, May 2, 1878, and the Causes of the same; by S. F. PECKHAM.

As I was sitting at the tea-table on the evening of May 2d, I was startled by a noise that sounded as if something as heavy as a barrel of flour had been tipped over on the floor above. A few seconds later the sound was repeated, and we all ran to the door which commanded a full view of the falls and manufacturing portion of the city. An immense volume of black smoke enveloped the spot where the Washburn A Mill had stood, and a perpendicular column of smoke was projected into the air above the elevator at least four hundred feet. The Humboldt and Diamond Mills were directly behind the elevator from the place where I stood. A heavy wind was blowing from a point a little to the east of north, a direction from the Washburn A Mill toward the elevator and the other two mills. In less than two minutes from the time of the first explosion, the elevator, which was 108 feet high, was wrapped in flames from top to bottom. If the structure had been saturated in oil the flames could not have spread much more rapidly. In five minutes flame and smoke were pouring from every window in the Day & Rollins, Zenith and Galaxy Mills, which were between the Washburn A Mill and the river, producing a conflagration which from ordinary causes would not have gained such headway in two hours. Six flouring mills, the elevator, a machine shop, blacksmith's shop and planing mill, with a number of empty and loaded cars, were in flames in five minutes from the time fire was first observed by any one who survived the disaster.

From my own point of observation, which was about a mile distant, but two distinct explosions were heard; others nearer heard three, the first not as violent as the other two; while those nearer still heard in addition a sound which they described as a succession of sharp hisses, resembling the sound of burning gun-powder. Those observers to the windward, whose attention was arrested by the light produced, beyond the distance of half a mile, heard only one or two reports or failed to hear any report at all. From all the testimony in reference to sound it appears that the blow upon the air was not sufficiently sudden to produce a penetrating sound, but rather a dull, heavy blow, which was not communicated laterally to any great distance.

Burning wheat or flour was smelled for several minutes before the explosion by persons in such a position that the wind would carry the odor to them. Smoke was also seen issuing from what was known as the exhaust flour-dust spout

of the Washburn A Mill for several minutes preceding the explosion.

At the instant the explosion occurred all observers agreed that the Washburn A Mill was brilliantly illuminated from basement to attic. The illumination was reflected from the water at and around the falls in such a manner as to remind one observer of the effect of a brilliant sunset. Another compared it to the reflection of sunlight from windows when the sun is near the horizon. Still another, who was crossing the lower bridge, had his attention called to what appeared to be a stream of fire, which as he described it, issued from a basement window and went back again. Immediately thereafter each floor above the basement became brilliantly illuminated, the light appearing simultaneously at all the windows, only an appreciable interval of time intervening as the stories ignited one after the other. Then the windows burst out, the walls cracked between the windows and fell, and the roof was projected into the air, followed by an immense volume of smoke and flame which ascended to an estimated height of from six to eight hundred feet. As the column of smoke was expanded and borne off upon the wind, brilliant flashes resembling lightning passed to and fro.

Two men, so near the Humboldt Mill that they were nearly buried by the falling rubbish, and on the opposite side from the Washburn A Mill, heard a loud report distinctly while the walls of the Humboldt Mill were still standing and at the same time were knocked down. Immediately after they saw flames issuing from the basement windows of the Humboldt Mill and at the same instant, before they could regain their feet, they experienced a second shock and miraculously escaped being buried beneath the falling walls.

The enormous and sudden displacement of air which fol lowed the explosion, and the tremendous force which was consequently exerted laterally, was shown in the condition of the round-house of the Chicago, Milwaukee, and St. Paul railroad, and the broken windows in all directions. The 1ound-house was a wooden structure about forty or fifty feet from the Diamond Mill. The sills were drawn out toward that mill until the building burst, letting a part of the roof fall in and leaving the sides standing at a sharp angle. Ordinary windows, and those of strong plate-glass on Washington avenue one-fourth of a mile distant, were projected into the street. Not only the glass but the sash went out bodily, particularly in the lower stories of the buildings. Persons on the river at the water's edge noticed a displacement of the water producing a wave estimated to be eighteen inches high, before they heard the report of the explosion.

Whole sheets of the corrugated iron with which the elevator was covered, measuring eight by two feet but quite thin, were picked up on the east side of the river more than two miles. distant, and pieces of six-inch flooring from two to ten feet long were carried to intermediate points.

An examination of the ruins of the several buildings showed that the walls of the Humboldt Miil lay upon those of the Diamond Mill, and those of the Diamond Mill upon those of the west end of the Washburn A Mill, showing that the buildings did not explode simultaneously but successively. The Washburn A Mill evidently exploded first from fire originating within it, and the high wind prevailing at the time carried the flame into the adjoining mills to the south and away from the mills next the river. There was enough burning middlings and flour thrown through the broken windows of the latter mills to set them on fire, but they did not explode. Some significance may attach to the fact that the three mills that exploded were all running with more or less open French middlings purifiers, while the three that did not explode had been shut down for several days. There is no question but that the French purifiers project a great deal more dust into the atmosphere of the mills than those that are enclosed, but I have no doubt that in any flouring mill sufficient dust accumulates upon beams and machinery to produce an explosive atmosphere if from any cause this dust is scattered into the air and flame is communicated to the mixture while the dust is suspended.

There was less than a barrel each of lard oil, lubricating oil and high-test kerosene in the Washburn A Mill at the time of the explosion.

There is absolutely no proof that any explosive material other than is produced in the manufacture of flour from wheat was in any one of the buildings destroyed, in the cars around them or in the neighborhood. The testimony of mill-wrights conclusively showed that fire produced by heated bearings is of such extremely rare occurrence in flouring mills as to practically exclude such a cause.* No suspicion of incendiarism has ever been expressed.

A slight fire, the effects of which were in no wise serious, occurred in the Washburn A Mill about three months before the explosion. It was discovered from the outside of the mill that smoke was issuing from a spout or conductor that discharged the air that was drawn through between the stones.

* These gentlemen concurred in the statement that the spindle which carries the stone had been known to become welded into the socket in which it revolved, stopping the stone. When asked if the friction produced a welding heat, one replied, "no, no where near it." It must be an example of perfect metallic contact, producing cohesion.