April 24, 1884] 



NATURE 



611 



Mr. Moss the title of this afternoon's lecture. But now that the 

 lime has come for me to approach the subject before you I find 

 myself conscious of some misgivings, and the misgivings are 

 founded upon this ground : that tlie suljject is not one that lends 

 itself easily to experimental demonstration before an audience. 

 Many of the experiments can only be made on a small scale and 

 ri'f|uire to be watched closely. However, by help of diagrams and 

 by not confining myself too closely to our special investigation but 

 dealing somewhat with the wider subject of dust in general, I 

 may hope to render myself and my subject intelligible if not 

 very entertaining. 



First of all, I draw no distinction between "dust" and 

 '■ smoke." It would be possible to draw such a distinction, but 

 it would hardly be in accordance with usage. Dust might be 

 defined as smoke which had settled, and the term smoke applied 

 to solid particles still suspended in the air. But at present the 

 term " smoke " is applied to solid particles produced by com- 

 bustion only, and "dust " to particles owing their floating existence 

 to some other cause. This is evidently an unessential distinc- 

 tion, and for the present I shall use either term without distinc- 

 tion, meaning, by dust or smoke, solid particles floating in the 

 air. Then "fog" : this differs from smoke only in the fact that 

 the particles are liquid instead of solid. And the three terms, 

 dust, smoke, and fog, come to much the same thing, only that 

 llie latter term is applied when the suspended particles are 

 liquid. I do not think, however, that we usually apply the 

 term "fog" when the liquid particles are pure water: we call 

 ii then mostly either mist or cloud. The name "fog," at any 

 rate in towns, carries with it the idea of a hideous, greasy com- 

 liound, consisting of smoke and mist and sulphur and filth, as 

 unlike the mists on a Highland mountain as a country meadow is 

 unlike a city slum. Nevertheless the finest cloud or mist that 

 ever existed consists simply of little globules of water sus- 

 pended in air, and thus for our present purpose differs in no 

 important respect from fog, dust, and smoke. A cloud or mist 

 i-^, in fact, fine water-dust. Rain is coarse water-dust formed 

 by the aggregation of smaller globules, and varying in fineness 

 from the Scotch mist to the tropical deluge. It has often been 

 asked how it is that clouds and mists are able to float about 

 w hen water is so much heavier (800 times heavier) than air. 

 The answer to this is easy. It depends on tlie resistance or 

 viscosity of fluids, and on the smallness of the particles con- 

 cerned. Bodies falling far through fluids acquire a "terminal 

 velocity," at which they are in stable equilibrium — their weight 

 being exactly equal to the resistance — and this terminal velocity 

 is greater for large particles than for small ; consequently we 

 have all sorts of rain velocity, depending on the size of the 

 drops ; and large particles of dust settle more quickly than small. 

 Cloud-spherules are falling therefore, but falling very slowly. 



To recognise the presence of dust in air there are two prin- 

 cipal tests : the first is the obvious one of looking at it with 

 plenty of light, the way one is accustomed to look for anything 

 else ; the other is a method of Mr. John Aitken's, viz. to observe 

 the condensation of water vapour. 



Take these in order. When a sunbeam enters a darkened 

 room through a chink, it is commonly said to be rendered visible 

 by the motes or dust particles dancing in it ; Ijut of course really 

 it is not the motes which make the sunbeam visible, but the sun- 

 beam the motes. A dust particle is illuminated like any other 

 solid screen, and is able to send a sufficient fraction of light to 

 our eyes to render itself visible. If there are no such particles 

 in the beam — nothing but clear, invisible air — then of course 

 nothing is seen, and the beam plunges on its way quite invisible 

 to us unless we place our eyes in its course. In other words, to 

 he visible, light must enter the eye. [A concentrated beam was 

 passed through an empty tube, and then ordinary air let in.] 



The other test, that of Mr. Aitken, dejiends on the condensa- 

 tion of steam. When a jet of steam finds itself in dusty air, it 

 condenses round each dust particle as a nucleus, and fonns the 

 white visible cloud jjopularly called steam. In the absence of 

 nuclei Mr. Aitken has shown that the steam cannot condense 

 until it is highly supersaturated, and that when it does it con- 

 denses straight into rain — that is, into large drops which fall. The 

 condensation of steam is a more delicate test for dust than is a beam 

 of light. A curious illustration ofthe action of nuclei in condensing 

 moisture has just occurred to me, in the experiment — well known 

 to children — of writing on a reasonably clean window-pane, with, 

 .say, a blunt wooden point, and then breathing on the glass : the 

 condensation of the breath renders the writing legible. No doubt 

 (he nuclei are partially wiped away by the writing, and the 



moisture will condense into larger drops with less light- 

 scattering power along the written lines than over the 

 general surface of the pane where the nuclei are plentiful 

 and the drops therefore numerous and minute. Mr. Aitken 

 points out that if the air were ever quite dustless, vapour could 

 not condense, but the air would gradually get into a horribly super- 

 saturated condition, soaking all our walls and clothes, dripping 

 from every leaf, and penetrating eveiywhere, instead of falling 

 in an honest shower, against which umbrellas and slate roofs are 

 some protection. But let us understand what sort of dust it is 

 which is necessary for this condensing process. It is not the 

 dust and smoke of towns, it is not the dust of a country road ; 

 all such particles as these are gross and large compared with 

 those which are able to act as condensers of moisture. The fine 

 dust of Mr. Aitken exists everywhere, even in the upper regions 

 of the atmosphere ; many of its particles are of ultra-micro- 

 scopic fineness ; one of them must exist in every raindrop, 

 nay, even in every spherule of a mist or cloud, but it is only 

 occasionally that one can find them with the microscope. It is 

 to such particles as these that we owe the blue of the sky, and 

 yet they are sufficiently gross and tangible to be capable of being 

 filtered out of the air by a packed mass of cotton-wool. Such 

 dust as this, then, we need never be afraid of being without. 

 Without it there could be no rain, and existence would be in- 

 supportable, perhaps impossible ; but it is not manufactured in 

 towns ; the sea makes it ; trees and wind make it ; but the kind 

 of dust made in towns rises only a few hundred yards or so into 

 the atmosphere, floating as a canopy or pall over those unfortu- 

 nate regions, and sinks and settles most of it as soon as the air 

 is quiet, but scarcely any of it ever rises into the upper regions 

 of the atmosphere at all. 



Dust, then, being so universally prevalent, what do I mean 

 by dust-free spaces ? how are such things possible ? and where 

 are they to be found? In 1870 Dr. Tyndall was examining 

 dusty air by means of a beam of light in which a spirit-lamp 

 happened to be burning, when he noticed that from the flame 

 there poured up torrents of apparently thick black smoke. He 

 could not think the flame was really smoky, but to make sure he 

 tried first a Bunsen gas-flame and then a hydrogen flame. They 

 all showed the same effect, and smoke was out of the question. 

 He then used a red-hot poker, a platinum wire ignited by an electric 

 current, and ultimately a flask of hot water, and he found that 

 from all warm bodies examined in dusty air by a beam of light 

 the up-streaming convection-currents were dark. Now of course 

 smoke would behave very dilferently. Dusty air itself is only a 

 kind of smoke, and it looks bright, and the thicker the smoke 

 the brighter it looks ; the blackness is simply the utter absence 

 of smoke ; there is nothing at all for the light to illuminate, and 

 accordingly we have the blankness of sheer invisibility. Here is 

 a flame burning under the beam, and, to show what real smoke 

 looks like, I will burn also this spirit-lamp filled with turpentine 

 instead of alcohol. Wky the convection-currents were free from 

 dust was unknown : Tyndall thought the dust was burnt and 

 consumed : Dr. Frankland thought it was simply evaporated. 



In 1881 Lord Rayleigh took the matter up, not feeling satisfied 

 with these explanations, and repeated the experiment very care- 

 fully. He noted several new points, and hit on the capital idea 

 of seeing what a cold body did. From the cold body the de- 

 scending current was just as dark and dust-free as from a warm 

 body. Combustion and evaporation explanations suffered their 

 death-blow. But he was unable to suggest any other explana- 

 tion in their room, and so the phenomenon remained curious and 

 unexplained. 



In this state Mr. Clark and I took the matterup last summer, and 

 critically examined all sorts of hypotheses that suggested them- 

 selves, Sir. Clark following up the phenomena experimentally with 

 great ingenuity and perseverance. One hypothesis after another 

 suggested itself, seemed hopeful for a time, but ultimately had to be 

 discarded. Some died quickly, others lingered long. In the 

 examination of one electrical hyjjothesis which suggested itself 

 we came across various curious phenomena which we hope still 

 to follow up.' It was some months before what we now believe 

 to be the true explanation began to dawn upon us. Meanwhile 

 we had acquired various new facts, and first and foremost we 

 found that the dark plane rising from a wann body was only the 

 upstreaming portion of a dust-free coat perpetually being renewed 



» For inst.-ince, the electric prtperties of cry.st.^Is csn be readily examined 

 in illtim'.imted dusty air ;"the dust grjws on them in little bushes and marks 

 out thoir poles and neutral regions, without any need for an electrometer. 

 Magnesia smoke answers capitally. 



