266 



NA TURE 



[Jan. 22, 1885 



England, and terribly uphill will be the retracement of 

 your steps. The old country has in many things made 

 experiments for you — experiments of which you may reap 

 the benefit, without repeating them, if you choose. The 

 experiment of Protection, which we have tried and aban- 

 doned, I dare not here mention except just by name ; but 

 1 dare mention the experiment we have tried only too 

 successfully, and by no means yet abandoned though we 

 groan under it — that of fouling the atmosphere, wherever 

 a large number of human beings have to live in it, to such 

 an extent that it is not fit to breathe. We have made .1 

 terrible mistake, and one that will take perhaps a century 

 to undo. Tax all the necessaries of life and it is a small 

 evil, for the tax may at any time by an Act of Parliament 

 be removed, but pollute the air in which a people have to 

 live and no one can see the end of the evil. You will 

 soon have towns here rivalling Liverpool and Glasgow 

 and Manchester in size, and some day London. Be 

 warned in time. 



However, in speaking of dust, I am not going to con- 

 fine myself to such artificial dust as is made in towns, 1 

 shall include everything which Tyndall means when he calls 

 it " the floating matter of the air," all diffused and floating 

 foreign atter, fine or coarse. But the term "floating" 

 is not free from possible misconception, and a better term 

 than floating is sinking. If the two sound antagonistic, 

 then floating was wrong. Foreign particles, whether solid 

 or liquid, are not floating, and cannot float, in air; they are 

 all necessarily sinking through it, and sinking at a well- 

 defined and fairly calculable rate. Consider, for instance, 

 the water globules of a fog, or mist, or cloud. The drops of 

 water appear to float in air, but they are not floating, they 

 are slowly settling down. They may in truth be buoyed 

 up by convection currents, but they never move up through 

 the air, they move up with the air to some extent, but are 

 always slowly falling through it whether the air be moving 

 or stationary. Are they then like .1 slowly-falling balloon 

 or soap-bubble ? No, they are not buoyed up at all — they 

 are falling as fast as ever they can. Water is 800 times as 

 heavy as air, and a drop of water falls under the influence 

 of this enormous difference in weight. Why does it not 

 fall faster? Just for the same reason as prevents an 

 Atlantic liner from being propelled at 50 knots an hour — 

 skin friction. A ship requires a great force to propel it 

 at 15 knots an hour ; break it up into small pieces and it 

 will take vastly more ; pound it into infinitely fine dust 

 and it will require an infinite force to propel it at any 

 slow pace. I do not say that a small body as it moves 

 through a fluid experiences more resistance than a large 

 one — it experiences less ; but the decrease of resistance is 

 not so rapid as th decrease of its bulk or weight, and 

 consequently t small filling body is resisted more in 

 portion ti> its :, ./ r ht than a large one. Consider a bullet 

 or a raindrop falling from a great height. As it fall- it 

 keeps moving quicker and quicker, but not without limit. 

 Its weight remains constant, the resistance it meets 

 with increases with its speed ; hence there comes a time 

 when the two balance and the body is in equilibrium. 

 It then ceases to gain speed : it has attained its " terminal 

 velocity." Even if thrown down faster than this it would 

 slacken till it attained it. Now this terminal velocity is 

 greater for a bullet than for a small shot, is greater for a 

 large raindrop than for a small one, and for a mist globule 

 is very small. The old idea concerning cloud globules, 

 that they were hollow vesicles and therefore floated, is 

 quite erroneous. They do not float, they sink. Slowly 

 sinking particles, then, constitute dust, whether these par- 

 ticles be solid or liquid. W iter dust is 50 important that 

 it has various names, such as mist, fog, rain, cloud, and, 

 in popular usage, steam. 



Having now stated what dust is, the question presents 

 itself, How did it get there ? What are the sources of dust ? 

 There are certain human sources of dust — such as the 

 traffic of towns, and the smoke of imperfect combustion. 



These produce coarse and heavy particles which never 

 rise to a great height, nor float very far from their source : 

 this dust may be regarded as mere dirt and filth. Be- 

 sides this, however, a fine impalpable dust is produced 

 by every terrestrial activity. The wind blowing through 

 trees, the waves tossing up spray — all these natural 

 activities disperse into the air very fine particles, which 

 are upborne and carried so far from their source that they 

 form quite a permanent part of the atmosphere. This 

 fine natural dust is not limited to the lower atmospheric 

 levels, but is almost equally abundant at great heights ; 

 to it we owe the blueness of the sky, and by it clouds and 

 mists are rendered possible. 



Another source of dust is found in volcanoes. During 

 an eruption immense torrents of pumice and ashes are 

 driven upwards to incredible heights, whence they 

 slowly settle down again, the larger fragments sometimes 

 covering the sea for acres with a thick floating deposit 

 through which steamers slowly crunch their way, almost 

 as if steering through land (see a graphic account in 

 Nature, signed Stanley M. Kendall, forwarded by Prof. 

 Turner, vol. xxx. p. 288, July 24, 1SS4 ; see also vol. xxix. 

 P- 375, abstract of paper by Capt. Vereker) ; the finer 

 particles being carried hundreds of miles away from their 

 source, and giving rise to brilliant appearances as they 

 catch the solar rays — appearances recently observed over 

 a great part of the world at sunset. 



Yet another variety of dust is that which comes to us 

 from ultra-terrestrial sources, fragments of interplanetary 

 matter, cosmic or meteoric dust. You all know of the 

 showers of falling stones— the August and November 

 meteors ; you know that these are lumps of interplanetary 

 matter careering through space, mostly doubtless round 

 the^sun, but not aggregated together into planets. Cold 

 lumps of iron they mostly seem to be, possibly fragments 

 of some ancient world, possibly relics of the old nebulous 

 world material, never yet aggregated into worlds at all. For 

 ages the}- may have been rushing along, some almost isola- 

 ted, others crowded together, and so they might rush on for 

 millions of years ; but a larger body bears down upon some 

 of them ; they feel the gravitative influence of the huge 

 mass of a planet ; they are deflected from their course 

 notwithstanding their prodigious speed, and a few dip 

 into its atmosphere. In an instant the terrific friction 

 strips off their outer coat, scrapes and rubs the surface till 

 it glows with a white heat ; streams of white-hot particles 

 are still scraped oft', and form a luminous trail, but the white- 

 hot masses plunge on : and one perhaps escapes to resume 

 it- wanderings, disturbed a little by its encounter but not de- 

 stroyed ; another may be rubbed to fragments altogether ; 

 another may be heated so rapidly and unequally as to 

 explode ; while another may enter the atmosphere at a 

 more moderate velocity — may be heated indeed, and 

 violently scraped, but not destroyed, and may embed itself 

 in the ground, to be dug up by some peasant as a thunder- 

 bolt and to be preserved in some museum. The frayed 

 particles of such meteors must constitute no inconsi- 

 derable portion of terrestrial dust ; and since it comes 

 from altogether c;tra-terrestrial sources, it is to us of 

 most intense interest. One other visitant from other 

 worlds we know of, and that is light. Light is found to 

 be charged with information, though it took man many 

 centuries to learn how to read it — first with the telescope, 

 now with the spectroscope, and next with who shall saj 

 what still more potent revealer and analyser of hidden 

 truth. Meteoric dust may not be so laden with informa- 

 mation as light is — certainly we have not yet learnt to 

 read it. It is only within the last few years that, at 

 the instigation of Sir William Thomson, a Committee oi 

 Section A of the British Association was appointed to 

 consider the question whether such dust could be col- 

 lected and detected at all. Under the able and energetic 

 guidance of Dr. Schuster, this Committee has done good 

 work, and some dust from the ice-fields of the Himalayas 



