Jan. 22, 1885] 



NA TURE 



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and from Greenland has been definitely proved to be 

 meteoric. 



W present however no sign of organic matter or evi- 

 dence of extra-terrestrial life has yet been detected in it, 

 but any year this statement may have to be modified, and 

 a discovery of the most intense interest may have to be 

 announced. You have probably all heard of this theory 

 of Sir William Thomson's, that some life germs may have 

 been carried to the earth by a meteor, and you are pro- 

 bably equally well acquainted with the cheap ridicule 

 the statement met with at the hands of newspaper article 

 writers and the general public. It was derided as an 

 absurd attempt to explain the origin of life. It was nothing 

 of the kind. Nothing at all was said about the origin of 

 life ; it was a sober matter-of-fact statement that it was a 

 scientific possibility for some organic germs or seeds to be 

 conveyed to the earth by a meteor, to be rubbed off it at 

 its first entrance into the atmosphere without getting 

 overheated, and thence to slowly settle down as dust, and 

 germinate. Well, it is a possibility, and it may before 

 now have happened, and it may happen again, and very 

 interesting it would be to be able to point to a case of its 

 happening. But what then? If you account for the 

 presence of a cherry-tree in your orchard by saying 

 that it sprang from a cherry-stone dropped by a pass- 

 ing balloon, are you to be assailed as a full-blown 

 explainer of the origin of all cherry-trees and of all forms 

 of life ? 



You may take it as a fairly safe rule that when a state- 

 ment is made by the highest living scientific authority, 

 the statement may or may not be true, but it is not likely 

 to be such abject nonsense that any newspaper article 

 writer, in the interval between ten o'clock and midnight, 

 can see all through it, detect its follies, and serve them up 

 exposed for your breakfast edification. 



Leaving the subject of meteoric dust now, and of the 

 possibility of future discovery which may be wrapped up 

 in it, let us proceed to ask, What is dust for — what pur- 

 pose does it serve ? We shall not enter upon the teleo- 

 logical inquiry, what was it intended to do ; we shall 

 simply ask what it does — a plainer, and for the most part 

 a more instructive, question. 



I :rst, what is the function of human dust, such as is 

 made in towns ? One of its functions is to choke up the 

 breathing organs both of plants and animals ; another 

 is to propagate disease from place to place. It is one of 

 the most, important discoveries of this century, that in- 

 fectious disease is due to the growth of a specific vege- 

 table organism in the system, propagating itself like yeast 

 in dough, or ferments in alcoholic liquors. The germs of 

 ihese organisms float about in the air from place to place, 

 and gain positions enabling them to enter the blood of 

 some animal organism, say man, where they can grow and 

 flourish, provided they are able to successfully encounter 

 1 heir mortal foes, the white corpuscles of the blood. If 

 bite corpuscles are strong and vigorous, they will 

 overpower the foreign growth, and kill it. If, on the other 

 hand, they are weak and feeble, and the germs are very 

 numerous, the foreign growth may get a secure footing and 

 spread luxuriantly, changing the character of the fluids of 

 the body, coagulating, it may be, the albumen, and other- 

 wise setting up the unnatural and abnormal display of 

 functions which we call disease. I have only to indicate 

 thus much to exhibit to you the enormous field of know- 

 ledge and of inquiry which is involved in the discussion 

 of the function of dust from this point of view. 



But it is not my province to discuss this, and I must 

 hasten on to more purely physical considerations, and 

 must ask, What is the function of the fine impalpable dust 

 or ultra-microscopic particles in the upper regions of the 

 air ? First of all, it is this which causes the blue of the 

 sky and the diffusedness of daylight. I have not time 

 to go into this. I will only state it, and pass on. You 

 will find the rudiments of it beautifully expressed by 



Dr. Tyndall in his Lectures on Light, but it will take 

 Lord Rayleigh to explain it to you completely. 1 



If the atmosphere were purely gaseous, and held 

 no minute foreign bodies in suspension, the aspect 

 of the sky would be utterly different from what it now 

 is. The sun would glare down directly with blinding 

 intensity, and objects not in direct sunlight would be 

 in almost complete shadow. A room facing north would 

 be m something like darkness : at least, it would be only 

 illuminated by reflection from illuminated objects outside. 

 The sun would be set in a black firmament, and if its 

 direct light were screened off it would be easy to see the 

 stars at noonday. (Through dust-free air light passes 

 on without loss by scattering, and is quite invisible except 

 to any eye placed directly in its course. [Tyndall's 

 optically empty tube was here shown.] There is nothing 

 remarkable in seeing nothing, when no dust or other 

 reflecting body is present. When you see motes dancing 

 in a sunbeam, it is not the motes which render the sun- 

 beam visible, but the sunbeam the motes ; and of course 

 light is invisible which does not enter the eye.) 



What is the actual state of things as contrasted with this ? 

 The sun's rays on reaching our atmosphere are partially 

 intercepted, diffused, and scattered by myriads of most 

 minute particles, so minute as to be even smaller than the 

 light-waves themselves, and to act on the smallest of 

 these waves more powerfully than on the largest. The 

 light thus scattered is the diffuse daylight so entirely 

 satisfactory and pleasant to the eye, and so inimitable by 

 artificial systems of illumination. The light thus scat- 

 tered has a preponderance of small waves, owing to the 

 minute size of the scattering particles, and hence it affects 

 our sight organ with the sensation of blue. By this scat- 

 tered light shadows are mellowed, the intensity of direct 

 sunlight is mitigated, and the whole expanse of sky glows 

 with a perfect lustre, effectually drowning the light from 

 the more distant celestial bodies. Above the top of a 

 high mountain dust is almost absent, and there the sky 

 has been observed at times to look almost black, and 

 stars are sometimes visible in sunlight. 



But besides the blue of the sky, we owe to this dust 

 the possibility of clouds, which still further intercept and 

 scatter the solar beams. " Cloud is visible vapour of 

 water floating at a certain height in the air," says Mr. 

 Ruskin - ; but he is not quite right in Ins language. True 

 vapour of water is invisible, and that which is visible is no- 

 longer vapour, but condensed vapour. It is vapour which 

 has condensed to liquid — not to great masses of liquid, but 

 to minute globules or spherules of liquid, so small as only to 

 sink very slowly through the air. V\ hat makes the vapour 

 condense into this water-dust form ? Why does it not 

 condense at once into great masses or sheets of water ? 

 Something there must be to start the condensation at multi- 

 tudes of separate points, so that the vapour shall condense 

 the instant it is saturated, without ever becoming super- 

 saturated. Things that act in this way are called nuclei. 

 Without a nucleus, it is as easy for a phenomenon to begin 

 at one place as at another, and when that is the case it 

 does not begin anywhere: there is no preponderating 

 cause. Wherever there is a nucleus, however, there the 

 action can lxg;n ; and in order that action may 1 ommence 

 at an infinity of points at once, it is necessary that an 

 infinity of nuclei exist. The action of nuclei is readily illus- 

 trated by the well-known experiment of a supersaturated 

 solution of Glauber's salts. The solution remains liquid 

 until a nucleus is introduced, when it becomes suddenly 

 converted into a solid. (I don't say that it is clear why 

 nuclei are able to start the action. What is there at the 

 surface of discontinuity to make change of state easier 

 there than anywhere else ? It will take a bigger man 

 than me to tell you that. 3 ) 



1 Phil. Mag., August 1SS1. * " Storm Cloud" lecture, p 12. 



J Sir W. Thomson has partially indicated a reason for it in I;: ol 



the effect of curvature of surface on vapour-tension. See Maxwell's "Heat." 

 chap. xx. p. 268. 



