March 20, 1890] 



NATURE 



473 



temperature with depth was found by Middendorff than would 

 have been found if the measurements had been made in a shaft 

 immediately after its being pierced. Nevertheless, the fact of 

 the frozen soil extending to a great depth, especially in the val- 

 ley of the Lena, is not to be contested ; nor can there be any 

 doubt as to the extension of frozen soil over large parts of 

 Siberia. M. Yatchevsky attempts to determine its limits from 

 general considerations about the average yearly temperature of 

 separate regions, and the thickness of their snow-covering ; and 

 he gives a map of the probable southern limits of the frozen soil 

 in Siberia, which do not differ much from the yearly isotherm of 

 - 2° C. It must, however, be remarked that though the map 

 approximately shows where the ever-frozen soil may be found 

 beneath the thin layer of soil which thaws every summer, it 

 ought not to be concluded that ever-frozen soil -will be found 

 everywhere within those limits. For instance, the granite rocks 

 on the surface of the Vitrin plateau being immediately covered 

 with immense marshes, the water from these marshes infiltrates 

 into the rocks, and, while the marshes are covered during the 

 winter with a crust of ice, their depths remain unfrozen. It 

 may thus be considered certain that immense spaces will be 

 found within the theoretical limits marked on the map, where 

 no ever-frozen soil will be discovered. The Russian Geogra- 

 phical Society is sending out a series of questions, in the hope of 

 obtaining accurate information, and it would be well if the same 

 thing were done in Canada. 



According to a letter from Iceland, dated Reykjavik, Feb- 

 ruary 5, 1890, a translation of which is printed in the current 

 number of the Board of Trade Journal, the population of Iceland 

 during the four years from 1885 to 1888 inclusive has diminished 

 by about 2400, the total number at the close of each of these 

 years having been, in 1885, 71,613 ; in 1886, 71,521 ; in 1887, 

 69,641 ; and in 1888, 69,224. This diminution was greatest 

 (1880) in 1887, the explanation for which may be sought in the 

 enormous emigration to America which took place in that year. 

 The diminution in the remaining years, though less sensible, 

 must be attributed to the same cause, as in these years the 

 number of births exceeded that of deaths. The chief diminution 

 has been shown by the northern and eastern districts. The 

 prefecture of Hunavatn in particular has fallen off in respect to 

 inhabitants from 4800 in 1885 to 3785 in 1888. In Reykjavik, 

 the capital, the population has risen from 3460 to 3599. 



ATMOSPHERIC DUST.^ 



'T'HE infinitely small particles of matter we call dust, though 

 possessed of a form and structure which escape the naked 

 eye, play, as you are doubtless aware, important parts in the 

 phenomena of nature. A certain kind of dust has the power of 

 decomposing organic bodies, and bringing about in them 

 definite changes known as putrefaction, while others exert a 

 baneful influence on health, and act as a source of infectious 

 diseases. Again, from its lightness and extreme mobility, dust 

 is a means of scattering solid matter over the earth. It may 

 float in the atmosphere as mud does in water, and blown by 

 the wind will perhaps travel thousands of miles before again 

 alighting on the earth. Thus Ehrenberg, in 1828, detected 

 in the air of Berlin the presence of organisms belonging to 

 African regions, and he found in the air of Portugal fragments 

 of Infusorise from the steppes of America. The smoke of the 

 burning of Chicago was, according to Mr. Clarence King 

 (Director of the United States Geological Survey), seen on the 

 Pacific coast. 



Dust is concerned in many interesting meteorological pheno- 

 mena, such as fogs, as it is generally admitted that fogs are 

 due to the deposit of moisture on atmospheric motes. Again, 

 the scattering of light depends on the presence of dust, and you 

 may remember ray showing you on a former occasion that 

 beautiful experiment of Tyndall, illustrating the disappearance 

 of a ray of light when made to travel through a glass receiver 

 free from dust, whilst reappearing as soon as dust is admitted 

 into the vessel. There is no atmosphere without dust, although 

 it varies largely in quantity, from the summit of the highest 

 mountain, where the least is found, to the low plains, at the 

 seaside level, where it occurs in the largest quantities. 



The origin of dust may be looked upon, without exaggeration. 



An Address delivered to the Royal Meteorological Societj', January 15, 

 1890, by Dr. William Marcet, F.R.S., President. 



as universal. Trees shed their bark and leaves, which are 

 powdered in dry weather and carried about by ever-varying 

 currents of air, plants dry up and crumble into dust, the skin of 

 man and animal is constantly shedding a dusty material of a 

 scaly form. The ground in dry weather, high roads under a 

 midsummer's sun, emit clouds of dust consisting of very fine 

 particles of earth. The fine river and desert sand, a species of 

 dust, is silica ground down into a fine powder under the action 

 of water. 



If the vegetable and mineral world crumbles into dust, on the 

 other hand it is highly probable that dust was the original state 

 of matter before the earth and heavenly bodies were formed ; 

 and here we enter the region of theory and probabilities. In a 

 science like meteorology, where a wide door is open to specu- 

 lation, we should avoid as much as possible stepping out of the 

 track of known facts ; still there is a limit to physical observa- 

 tion, and in some cases we can do no more than glance inlo the 

 possible or probable source of natural phenomena. Are we on 

 this account to give up inquiring for causes ? This question I 

 shall beg to leave you to decide, but where we have such an expe- 

 rienced authority as Norman Lockyer, I think the weight attached 

 to possibilities and theories is sufficiently great to warant my 

 drawing your attention for a few moments to the probable origin 

 of the stars and of our earth. 



I dare say many of you have read the interesting article in the 

 Nineteenth Century of November last, by Norman Lockyer, and 

 entitled "The History of a Star." The author proposes to 

 clear in our imagination a limited part of space, and then set 

 possible causes to work ; that dark void will sooner or later be 

 filled with some form of matter so fine that it is impossible to 

 give it a chemical name, but the matter will eventually condense 

 into a kind of dust mixed with hydrogen gas, and constitute 

 what are called nebulce. These nebulae are found by spectrum 

 analysis to be made up of known substances, which are mag- 

 nesium, carbon, oxygen, iron, silicon, and sulphur. Fortunately 

 for persons interested in such inquiries, this dust comes down ta 

 us in a tangible form. Not only have we dust shed from the sky 

 on the earth, but large masses, magnificent specimens of meteor- 

 ites which have fallen from the heavens at different times, some 

 of them weighing tons, may be submitted to examination. 

 From the spectroscopic analysis of the dust of meteorites we find 

 that in addition to hydrogen their chief constituents are mag- 

 nesium, iron, silicon, oxygen, and sulphur. 



There are swarms of dust travelling through space, and 

 their motion may be gigantic. We know, for instance, some 

 stars to be moving so quickly that, from Sir Robert Ball's 

 calculations, one among them would travel from London to 

 Pekin in something like two minutes. From photographs takett 

 of the stars and nebulae, we are entitled to conclude that the 

 swarms of dust meet and interlace each other, becoming raised 

 from friction and collision to a very high temperature, and 

 giving rise to what looks like a star. The light would last so 

 long as the swarms collide, but would go out should the 

 collision fail ; or, again, such a source of supply of heat may be 

 withdrawn by the complete passage of one stream of dust- 

 swarms through another. We shall, therefore, have various 

 bodies in the heavens, suddenly or gradually increasing or de- 

 creasing in brightness, quite irregularly, unlike those other 

 bodies where we get a periodical variation in consequence of the 

 revolution of one of them round the other. Hence, as Norman 

 Lockyer expresses it clearly, " it cannot be too strongly insisted 

 upon that the chief among the new ideas introduced by the 

 recent work is that a great many stars are not stars like the sun, 

 but simply collections of meteorites, the particles of which may 

 be probably thirty, forty, or fifty miles' apart." 



The swarms of dust referred to above undergo condensation 

 by attraction or gravitation ; they will become hotter and 

 brighter as their volume decreases, and we shall pass from the 

 nebulae to what we call true stars. 



The author of the paper I am quoting from imagines such 

 condensed masses of meteoric dust being pelted or bombarded 

 by meteoric material, producing heat and light, which effect will 

 continue so long as the pelting is kept up. To this circumstance 

 is due the formation of stars like suns. Our earth originally 

 belonged to that class of heavenly bodies, but from a subsequent 

 process of cooling assumed its present character. 



While apologizing for this digression into extra-atmospheric 

 dust, I shall propose to divide atmospheric dust into organic, 

 or combustible, and mineral, or incombustible. The dust scat- 

 tered everywhere in the atmosphere, and which is lighted up in 



