NATURE 



[_7cin. 



well on in hi<; ninety-seventh year. Hi; long-continued and 

 laborious researches in Iht zooloj>y, pal.eontology, anlhropolojjy, 

 and antiquities of liis native land gave liim a high place among 

 men of science. Among a lio^t of mi lor contributions he was 

 the author of a standard \\ ork on the Scandinavian fauna ; l)ut 

 that by which he was best known to us is the book of which the 

 English Iransla'-ion, edited by Sir John Lubbock, bears the title 

 of "The Primitive Inhabitants of Scandinavia; an Es«ay on 

 Comparative Ethnography, and a Contribution to the History of 

 the Development of Mankind." 



Tlie niimlier of our ordinary members has been fairly kept 

 up, the addi-ions liy election having slightly exceeded the losses 

 hy death and resignation ; but a larger increase in the future 

 will he necessary in order to carry on the operations of the 

 Institute in a successful manner, especially under the new con- 

 ditions to which I shall have to advert presently. Even by the 

 most careful management our treasurer has not succeeded in 

 bringing the expenditure of the year quite within our oidinary 

 incom:. 



The journal, I am glad to report, has been brought out with 

 exemplary punctuality, under the able and energetic supervision 

 of our director, Mr. Rudler. To this part of our operations I 

 think we may look with unmixed satisfaction, the number, 

 character, and variety of the communications contained in it 

 being quite equal to those of former years. 



With regard to our future, the next year will probably be one 

 of the most momentous in our annals, as we have determined 

 upon a great step, no less than a change of domicile. It was 

 ascertained in the course of last summer that we could only 

 remain in our present quarters at an increased rent upon that 

 which we had hitherto paid, and we therefore considered 

 whether it would be possible to obtain as good or better accom- 

 modation elsewhere. It happened fortunately that the Zoo- 

 logical Society was about to move into new freehold preuii es 

 at No. 3, Hanover Square, and would have spare rooms avail- 

 able for the occupation of other >ocieties. A committee of the 

 Council was appointed to examine and report upon the desira- 

 bility of moving, and negotiati ms were entered into with the 

 Council of the Zoological Society which have ended in our 

 becoming their tenants for the future. We shall have for the 

 purposes of our library, oflice, and Council meetings, two con- 

 venient rooms on the second floor immediately above ihe 

 library of the Zoological Society, ai d for the purpose of storing 

 cur stock of publications a small room on the basement. We 

 shall also have the use of a fir more handsome and commodious 

 meeting room than that which we occupy at the present moment, 

 and in a situation which is in many respects more advantageous. 

 Let us trust that this change may be the inauguration of an era 

 of prosperity to the In-titute, and of increased scientific activity 

 among its members. 



THE FORMATION OF SMALL CLEAR SPACES 

 IN DUSTY AIR^ 



fN the introduction a few remarks are made en the growing 

 interest in everything connected with dust, whether it be the 

 organic germs floating in the air, or the inorganic particles that 

 pollute our atmosphere. Prof Tyndall's observations on the 

 dark plane seen over a hot wire - are referred to. Lord Uay- 

 leigh's recent discovery of the dark plane formed under a cold 

 body ^ is described, and attention called to Dr. Lodge's experi- 

 ments described in a letter to Nature, vol. xxviii. p. 297. 



The experiments described in this paper weie made in a small 

 dust-box, blackened inside, glazed in front, and provided with a 

 window atone side. P'or illumination two jtts of gas inclosed in 

 a dark lantern were u-ed. The light entered the dust-box by 

 the side window and could be condensed on any part of the 

 inside of the box, by means of two lenses fixed in a short tube, 

 and loosely attached to the front of the lantern. Magnifying 

 glasses of different powers were used for observation. The dusls 

 experimented on were made, some of hydrochloric acid and 

 ammonia, some by burning sulphur and adding ammonia, some 

 by burning paper, magnesium, or sodium. Calcined magnesia 

 and lime were also used, as well as ground charcoal. These 

 three last substances were stirred ujj by means of a jet of air. 



' Abstract of a paper read to Ihe Royal Society of Edinburgh, January 21, 

 i88.». by .Mr. John Aitken. 



" '■ E: ays en the Floating-Matter in the Air," p. 5. (Longmans, Green, 

 and Co., 1881.) 



For testing the effects of slight difference of temperature, tubes 

 in s^ine form or other were generally used. These tubes were 

 closed at the front, | rojec'.ed through the back of the dust-box, 

 and were brought cl »e to the glass front for observation under 

 strong magnifying powe'. The tubes were heated or cooled by 

 circulating water through them, in a small tube pas^ing through 

 their interior. 



Suppose the ex|)eriments to be begun 'by introducing a R.und 

 tube into its place in the dust-box and then filling the box with 

 any du^t, everything btirg then left for some time so that all the 

 apparatus may acquire the same temperature. If the lijjht be 

 now allowed to fall on the Vjox, and be quickly brought to a 

 focus on the tube, it will be found that the dust is in close con- 

 tact with it on the top and sides, but uuJcnimth there will he 

 seen a clear space. Close examination w ill show the 1 articles 

 to be falling on the upper surface of the tube, and coming into 

 contact with it, while underneath a clear space is formed by the 

 particles falling out of it. If the tube is now slightly cooled, a 

 downward cnrreiit is formed, and the currents of dustless air 

 from below the tube meet under it, and form a dark plane in the 

 centre of the descending current. It is shown that gravitation 

 can, under favourable conditions, produce this separation of the 

 dust quickly enough to keep up a constant supply of dustless 

 air. No increase of effect is produced by a lower temperature. 

 A temperature of - 10" C. makes the dark plane thinner, be- 

 cause it increases the rate of the descending current and carries 

 away the purified air more quickly. 



A form of apparatus was arranged to get rid of this separating 

 effect of gravitation. It consisted of an extremely thin and flat 

 piece of metal. This test-surface was placed ver/Ually in the 

 dust-box. The air in pa-ising over this piece of apparatus was 

 not caused to take up a horizontal movement at any part of its 

 passage. The result was that even with a temperature of 

 - 10" C. the dust kept close to its surface, and no dark plane 

 was formed in the descending current. The dark plane in the 

 cold descending current seems, therefore, not to be an effect of 

 temperature, but is the result of the action of gravitation on the 

 ]5articles under the body. A dark plane was, however, observed 

 when working with this flat surface when cooled, but it was not 

 formed in du^ty, but in foggy air, and was found to be due to 

 the evaporation of the fog particles when ihey approached the 

 cold surface. 



If a very little heat, instead of cold, is applied to the round 

 tube in the previous experiment, then the dark space under the 

 tube rises and encircles the tube and the two currents of clear 

 air unite over the tube and form the dark plane in^the upward 

 current. But in addition to this heat has been found to exert a 

 repelling effect on the dust. This was prove! by putting the 

 thin vertical test surface in the dust-box and heating it, when it 

 was found that the dust was repelled from its surface, and a 

 dark plane formed in the ascending current, neither of which 

 effects was obtained with cold. The dust begins to be repelled 

 with the sl'ghlest rise of temperature, and the dark space in 

 front of the test-surface becomes thicker as the temperature rises. 

 An experiment is then decribed in which air flowing up between 

 wo parallel glass plates is caused to pass from side to side 

 of the chaimel by the repelling action of heat at different 

 points. 



Fur testing the effects of higher temperatures a platinum wire 

 heated by mi.ans of a battery vas used. The (ilatinum wire was 

 bent into a U -shape, the two legs being brought close together. 

 This wire was fixed in the dust-box with the bend to the front, 

 and the legs iia^flie same horizontal plane, the two copper 

 wires to which it was attached beij g carried back\\ards and out 

 of the box. By this arrangement a clear view was obtained all 

 round the wire, and other advantages secured. Experimenting 

 w'i h this apparatus it was found that every kind of dust had a 

 different sized dark plane. Willi magnesia and other inde-truc- 

 tible dusts it was very thin, with the sulphate dust it was much 

 thicker, and with the sal-ammon'ac du t thicker still. So thick 

 was it with the two latter kinds of du^t that the dark planes over 

 the two legs expanded and foimed one plane. As the particles 

 could be seen streaming into the dark space under the wires, it 

 was obvious that these large dark plai es were not caused by 

 repul-ion, but by the evaporation or by the disii.tegration of the 

 dust particles. When making the experiment in a mixture of 

 different kinds of dusts, the hot wire was surrounded by a series 

 of zones of different brightness, and having sharp outlines. The 

 size of the diflicrent zones v as determined by the temperature 

 necessary to evaporate the different kinds of dust present, and 



