;24 



NA TURE 



[Jan. 31, 1884 



for trapping sont in chimneys. A small trap of this kind was 

 shown. It consisted of a fall metal lube or chimney, .surrnunded 

 liy another tube slightly larger. The products of combustion 

 are taken up the centre tube .nnd down the intervening space. 

 The heat of the "a^cs is thus made to do its own filtering. 

 This apparatus being placed over a smoky lamp, it trapped out 

 most of the soot, and deposited it on the inwde of the outer 

 tube. This arrangement of apparatus is too delicate and trouble- 

 some for general use, and it is suggested that, as by simply 

 cooling gases in presence of plenty or surface much of its dust 

 is deposited, it might be possible and advantageous under 

 certain conditions to purify air by heating and cooling it a 

 number of times, which could be done at a small expense by 

 means of regenerators. 



Experiments were also made by discharging electricity into 

 the smoke in a chimney. This also produced a marked diminu- 

 tion in the blackness of the escaping smoke. The supply of 

 electricity of sufficiently high potential is however a difficulty 

 for the present. 



A VAST DUST ENVELOPE^ 



C CIENTIFIC men have evinced extraordinary interest in the 

 '-' wonderfully brilliant sunsets that have for some time pa^t 

 been observed in different parts of the w-orld. Various theories 

 have been advanced, but all are agreed that the real cause is not 

 yet definitely determined. At the I>l-evo^rt House yesterday, a 

 Tribune reporter spent a couple of hours with Prof. S. V. 

 Langley, astroiomer at Allegheny Observatory, Allegheny, Penn. 

 His views upon the topic of the transmissibility of light through 

 our atmosphere are stated below : 



" At first I supposed the sunset matter a local phenomenon, 

 but when the reports showed it to have been visible all over the 

 world, it was obvious that we must look for some equally general 

 cause. We know but two likely ones, and the-e have been 

 already brought forward. One is the advent of an unusual 

 amount of meteoric dust. While something over ten millions of 

 meteorites are known to enter our atmosph(?re daily, which are 

 dissipated in dust and vapour in the upper atmosphere, the total 

 mass of these is small as compar. d with the bulk of the atnin- 

 sphere itself, although absolutely large. It is difficult to state 

 with precision what this amount is. Hut severa' lines of evidence 

 lead us to think it is approximately not gi-eatly less than 100 

 tons per diem, nor greatly more than 1,000 tons per diem. 

 Taking the largest estimate as still below the truth, we must 

 suppose an enormously greater accession than this to supply 

 quantity sufficient to produce the phenomenon in question ; and 

 it is hardly possible to imagine such a meteoric inflow unaccom- 

 panied with visual phenomena in the f rm of 'shooting siars,' 

 which would make its advent visible to all. Admitting, then, 

 the possibility of mete iric influence, we must consider it to be 

 nevertheless extremely improbable. 



" There is another cause, which I understand has been sug- 

 gested by Mr. Lockyev — though I have not seen his article — 

 which seems to be more acceptable — that of volcanic dust ; and 

 in relation to this presence of dust in the entire atmosphere of 

 the planet, I can offer some little personal experience. In 187S 

 I was on the upper slopes of Mount Etna, in the volcanic wa-tes, 

 three or four hours' journey above the zone of fertile ground. I 

 p-issed a portion of the winter at that elevation engaged in 

 studying the transparency of the earth's atmosphere. I was 

 much impressed by the fact that here, on a site where the air is 

 supposed to be as clear as anywhere in the world, at this con- 

 siderable altitude, and where we were surrounded by snow-field- 

 and deserts of black lava, the telescope showed that the air was 

 filled w ith minute dust particles, wdiich evidently had no rclatiin 

 to the Ijcal surroundings, Ijut apparently formed a portion of an 

 envelope common to the whole earth. I was confirmed in this 

 opinion by my recollection that Prof. Piazzi Smyth, on the Peak 

 of Teneriffe, in mid-ocean, saw these strata of dust rising to the 

 height of over a mile, reaching out to the horizon in every 

 direction, and so dense that they frequently hid a neigh'iouring 

 island mountain, whose peak rose above them, as though out of 

 an upper sea. In 1881 I was on Mount Whitney, in Southern 

 California, the highest peak in the Uniied States, unless some 

 of the Alaskan mountains can rival it. I had gone there with 



' From the New York Daily Trilti 

 rof. Piazzi Smyth. 



, January 2. Communicated by 



an expedition from the Allegheny Observatory, under the offki.il 

 direction of General Ilazen, of the .Signal Service, and had 

 camped at an altiiude of 12,000 feet, with a special object of 

 studying analogous phenomena. On ascending the peak of 

 Whitney, from an altitude of nearly 15,000 feet the eye boks 

 to the east over one of the most barren regions in the world. 

 Immediately at the foot of the mountain is the Inyo Desert, aid 

 on the east a range of mountains parallel to the Sierra Nevada-, 

 but only about 10,000 feet in height. From the valley the 

 atmosphere had appeared beautifully clear. But from thi- 

 aerial height we looked down on what seemed a kind of level 

 dust-ocean, invisible from below, but whose depth was six 01 

 seven thousand feet, as the upcer portion only of the opuosite 

 mountain range rose clearly out of it. The colour of the light 

 reflected to us from this dust-ocean was clearly red, and il 

 stretched as far as the eye could reach in every direction, 

 although there w-as no special wind or local cause for it. It wa- 

 evidently like the dust seen in mid-ocean from the Peak of 

 Teneriffe — something present all the time, and a permanent 

 ingredient in the earth's atmosphere. 



" At our own great elevation the sky was of a remarkably deei> 

 violet, and il seemed at fir t as if no dust was present in this 

 upper air, but in getting, just at noon, in the edge of tbchadov. 

 of a ran^e of cliffy which rose 1,200 feet above us, the sky im- 

 mediately about the sun took on a whitish hue. On scrutinising 

 this through the telescope it w.as Ibund to be due to myriads of 

 the minutest dust particles. 1 was here at a far greater height 

 than the suaimit of Etna, with nothing around me except granite 

 and snow-fields, and the presence of this dust in a comparatively 

 calm air much impressed me. I mentioned it to Mr. Clarence 

 King, then Director of the Uniied States Geologcal Survey-, 

 who was one of the first to ascend Mount Whitney, and he 

 informed me that this upper dust was | robably due to the ' loess ' 

 of China, having been borne across the Pacific and a quarter of 

 the way around the world. We were at the summit of the con- 

 tinent, and the air which swept by us was unmingled with that 

 of the lower regions of the earth's surface. Even at this great 

 altitude the dust was perpetually present in the air, a d I betam< 

 confirmed in the opinion that there is a permanent dust shell 

 inclosing the whole planet to a height certainly of abut three 

 miles (where direct observation has followed it), and not improb- 

 ably to a height even greater ; for we have no reason to suppose 

 that the du^t carried up from the earth's surface stops at the 

 height to which we have a-cended. The meteorite , which arc 

 consumed at an average height of twenty to forty miles, must 

 add somewhat to this. Our observations with special appar.ilus 

 on Mount Whitney went to show that the red rays are trans- 

 mitted with greatest facility through our air and rendered i- 

 extremely probable thit this has a very large share in Ihe colour- 

 of a cloudless sky at sunset and sunrise, these colours depending 

 largely upon the average size of the dust panicles. 



" It is especially worth notice that, as far as such observation, 

 go, we have no reason to doubt that the finer du t from the 

 earth's surface is ccirried up to a surprising altitude. I speak 

 here, not of the grosser dust particles, but of tho-e which are 

 so fine as to be individually invisible, except under favouring 

 circumstances, and which are so minute that they might be an 

 almost unlimited time in ■ettling to the ground, even if the atmo- 

 sphere were to become perfectly quiet. I have not at hand any 

 data for estimating the amount of dust thrown into the air by such 

 eruptions as these which recently occurred in Java and Alaska. 

 But it is quite certain, if the accounts we have are not exaggerated, 

 that the former alone must have been counted by millions of tons 

 and must in all probability have exceeded in amount that con- 

 tributed by meteorites during an entire year. Neither must it 

 be supposed that this will at once sink to the .surface again. 

 Even the smoke of a conflagration so utterly insignificant, com- 

 pared with nature'.s scale, as the burning of Chicago, was, 

 according to Mr. Clarence King, perceived on the Pacific Coast; 

 nor is there any improbability that I can see in supposing that 

 the eruption at Krakatoa may have charged the at uosphere of 

 the whole planet (or at least of a belt encircling it) for months 

 with particles sufficiently large to scatter the rays of red light 

 and partially absorb the others, and to produce the phenomenon 

 that is now exciting so much iiublic interest. We must not con- 

 clude that the cause of the phenomenon is certainly known. It 

 is not. But I am inclined to think that there is not only no 

 antecedent improbability that these volcanic eruptions on such 

 an unprecedented scale are the cause, but that they are the most 

 likelv cause which we can assi jn." 



