תמונות בעמוד
PDF
ePub

Yet, wrapped in a soiled paper, in the In all his thought, for all those years, pocket of his frock-coat, after death, they she was his great dear friend. Once or found a girl's likeness. “My photograph, twice he had held her beautiful handsbecause I trust you !" she had said to him looked at her eyes—been strong and happy at Aix-les-Bains. And what was all the in the magnetism of her presence.- Fortrest!

nightly Review.

DUST.

BY DR. J. G. MCPHERSON, F. R. S.E.

Some of the most enchanting phenomena discern them, they at times sorely tax the in nature are dependent for their very patience of the tidy housekeeper and the existence upon singularly unimportant skill of the anxious surgeon. An æsthetic things ; and some phenomena that in one eye is charmed with their gorgeous transforın or another daily attract our attention formation effects ; yet some are more real are produced by startlingly overlooked ma emissaries of evil than poet or painter ever terial. What is the agent that magically conceived. transforms the leaden heavens into the Until the famous discovery made by Mr. gorgeous afterglow of autumn, when the John Aitken, of Falkirk, a few years ago, varied and evanescent colors chase each no one could reasonably account for the other in fantastic brilliancy? What is the existence of rain. It was said by physicists source of the beautiful, brilliant, and varied that cloud-particles were attracted by the coloring of the waters of the Mediterranean, law of gravitation under certain conditions or of the most extraordinary brilliant blue of of temperature and pressure. But this the crystal waters of the tarns in the Cordil. famous experimentalist and observer found leras ? What produces the awe-inspiring out that without dust there could be no deep blue of the zenith in a clear summer rain ; there would be nothing but continuevening, when the eye tries to reach the ous dew. Our bodies and roads would be absolute ? Whence come the gentle re- always wet. There would be no need for freshing rain, the biting sleet, the stupefy- umbrellas, and the housekeeper's temper ing fog, the chilling mist, the virgin snow, would be sorely tried with the dripping the glimmering haze, or the pelting hail ! walls. What raises water to the state of ebullition A very easy experiment will show that in the process of heat application for boil- where there is no dust there can be no fog. ing? What is the source of much of the If common air be driven through a filter wound putrefaction, and the generation of cotton-wool into an exhausted glass reand spread of sickness and disease ? ceiver, the vessel contains pure air without What, in fact, is one of the most marvel- dust, the dust having been seized by the lous agents in producing beauty for the cotton-wool. If a vessel containing comeye's gratification, refreshment to the arid mon air be placed beside it, the eye is soil, sickness and death to the frame of unable to detect any difference in the conman and beast? That agent is dust. tents of the vessels, so very fine and in

And yet no significance is given to dust visible is the dust. If both vessels be conunless it appears in large and troublesome nected with a boiler by means of pipes, quantities." It requires the persistent an- and steam be passed into both, the obnoyance of dust-clouds to excite any at server will be astonished at the contrast tention. Dust, however, demands to be presented. In the vessel containing comnoticed, even when not in that collected, mon air the steam will be seen, as soon as irritating motion known in Scotland as it enters, rising in a close white cloud ; stour. The dust-particles floating in the then a beautiful foggy mass will fill the atmosphere or suspended in the water have vessel, so dense that it cannot be seen a most important influence upon the im- through. On the other hand, in the vessel agination, as well as upon the comfort of containing the filtered, dustless air the

Though so small that a microscope steam is not seen at all; though the eye magnifying 1600 diameters is required to be strained, no particles of moisture are

man.

discernible; there is no cloudiness what- of the water vapour, and mist-particles or ever. In the one case, where there was even rain-particles will be formed. the ordinary air impregnated with invisible This principle that every fog-particle has dust, fog at once appeared ; whereas in the embosomed in it an invisible dust-particle other case, the absence of the dust pre- led Mr. Aitken to one of the most startling vented the water-vapor from condensing discoveries of our day—the enumeration into fog. Invisible dust, then, is required of the dust-particles of the air. Thirty in the air for the production of fog, cloud, years ago M. Pasteur succeeded in countmist, snow, sleet, hail, haze, and rain, ac- ing the organic particles in the air ; these cording to the temperature and pressure are comparatively few, whereas the numof the air.

ber of inorganic particles is legion. Dr. The old theory of particles of water- Koch, Dr. Percy Frankland and others vapor combining with each other to form have devoted considerable attention to the a cloud-particle is now exploded. Dust is enumeration of the micro-organisms in the required as a free-surface on which the air, and Mr. A. Wynter Blyth, the public vapor-particles will condense. The fine analyst in London, has done good service particles of dust in the air attract the in counting the micro-organisms in the vapor-particles and form fog-particles. different kinds of water in the vicinity. When there is abundance of dust in the Marvellous as are the results, still the procair, and little water-vapor present, there is ess was comparatively easy. By generatan over proportion of dust-particles ; and ing the colonies in a prepared gelatine, the the fog-particles are, in consequence, number of microbes can be easily ascerclosely packed, but light in form and small tained. in size, taking the more flimsy appearance But to attempt to count the inorganic of fog. But if the dust-particles are fewer dust seemed almost equal in audacity to in proportion to the number of molecules the scaling of the heavens. The numberof water-vapor, each particle soon gets ing of the dust of the air, like the numweighted, becomes visible, and falls in bering of the hairs of the head, was conmist or rain.

sidered as one of the prerogatives of the This can be shown by experiment. Let Deity. Yet Mr. Aitken has counted the a jet of steam be passed into a glass re gay motes that people the sunbeams." ceiver containing common air, and it will Though he could not enlarge the particles be soon filled with dense fog. Shut off by a nutritive process, as in the case of the the steam, and allow the fog to settle. organic particles, he has been able to en. The air again becomes clear. Admit large them by transferring them into fogmore steam, and the water-particles will particles, so as to be within the possibility seize hold of the dust-particles that previ- of accurate enumeration. His plan is to ously escaped. Fog will be formed, but dilute a definite small quantity of common it will not be so dense. Again, shut off air with a fixed large quantity of filtered, the steam, and allow the fog to settle and dustless air, and allow the mixture to be the air to clear. Then admit some steam, super-saturated by water-vapor ; the few and very likely the condensed vapor will particles of dust seize the moisture, befall as rain. If the experiment be often come visible in drops, fall on a divided enough repeated, rain instead of fog will plate, and are there counted by means of a be formed, because there are comparatively magnifying-glass. few solid particles on which the moisture The instrument employed by Mr. Aitken can condense. When, then, dust is pres- has taken various forms ; in fact, he has ent in large quantities, the condensed so far improved it that it can be carried in vapor produces a fug ; there are so many the coat-pocket. But the original instruparticles of dust to which the vapor can ment, which we saw and used, is most adhere that each can only get a very small easily described without the aid of diashare-s0 small, in fact, that the weight grams. But, instead of his decimal sysof the dust is scarcely affected by the ad- tem of measurements, we will use the dition of the vapor—and the fog formed ordinary system, that the dimensions may remains for a time suspended in the air, be more easily grasped by the general too light to fall to the ground. But when reader. Into a conimon glass flask of the number of dust-particles is fewer, each carafe-shape, and flat-bottomed, of 30 particle can take hold of a greater space cubic inches capacity, are passed two

small tubes, at the end of one of which is dust-particles in the whole is 40 times attached a square silver table, one inch 500 = 20,000 ; that is, there are 20,000 long. A little water having been inserted, dust-particles in the small quantity of comthe flask is inverted, and the table is placed mon air (one tenth of a cubic inch) which exactly one inch from the inverted bottom, was introduced for examination ; in other so that the contents of the air above the words, a cubic inch of that air contains table and below the bottom are one cubic 200,000 dust-particles-nearly a quarter inch. The observing table has been di- of a million. vided into a hundred equal squares, and By this process Mr. Aitken has been is highly polished, with the burnishing all able to count 74 millions of dust-particles in one direction, so that during the ob- in one cubic inch of the ordinary air of servations it appears dark, when the fine Glasgow. We counted with him 4 millmist-particles, falling on it, glisten opal- ions in a cubic inch of the air outside of like with the reflected light, in order that the Royal Society Rooms, Princes Street, they may be more easily counted. The Edinburgh. Inside the Room, after the tube to which the silver table is attached Fellows had met for two hours, on a winis connected with two stop-cocks, one of ter evening—the fire and gas having been which can admit a small measured portion burning for a considerable time we found of the air to be examined. The other tube 64 millions in a cubic inch of the air four in the flask is connected with an exhausting feet from the floor ; but near the ceiling syringe, of 10 cubic inches capacity. Over no fewer than 574 millions were counted the flask is placed a covering colored black in the cubic inch. He counted in one in the inside. In the top of this cover is cubic inch of air immediately above a inserted a powerful magnifying-glass, Bunsen flame the fabulous number of through which the particles on the silver 489 millions of dust-particles. The lowtable can be easily seen and counted. A esi number he ever counted was at Lucerne, little to the side of this magnifier is an in Switzerland : 3500 in the cubic inch. opening in the cover, tbrough which light on the summit of Ben Nevis the observer, is concentrated on the silver table. This using Mr. Aitken's apparatus, counted light, again, has had to pass through a from 214,400 down to 840 in the cubic spherical globe of water, in order to ab- inch. But on the morning of the 21st of stract the heat rays, which might vitiate July last there was a most marvellous obthe observations.

servation made. Though at the sea-level To perform the experiment, the air in the wind was steady, and the thermometer the flask is exhausted by the syringe. The did not vary, at the summit the wind sudflask is then filled with pure filtered air. denly veered round to the opposite direcOne-tenth of a cubic inch of the air to be tion of that below, blowing out of a examined is then introduced into the flask, cyclone, and the temperature rose ten deand mixed with the 30 cubic inches of grees. In consequence the extraordinarily dustless air. After one stroke of the low mean of only 34 dust-particles to the syringe this mixed air is made to occupy cubic inch was observed. an additional space of 10 cubic inches ; and We now come to the most pleasant of this rarefying of the air so chills it that the investigations in connection with dust. condensation of the water vapor takes place The very brilliant sunsets wbich began in on the dust-particles. The observer, look- the autumn of 1883, and continued during ing through the magnifying-glass upon the successive seasons with gradually decreassilver table, sees the mist-particles fall like ing grandeur, have arrested the attention of an opal shower on the table, and counts the physicist as well as of the general obthe number on a single square in two or server. What is the cause of the brilliant three places, striking an average in his coloring in these remarkable sunsets ! mind. Suppose the average number upon What is the source of the immense wealth one of these squares were five, then on the of the various shades of red which have whole table there would be 500 ; and these been so universally admired ? Gazing on 500 mist-particles contain the 500 dust- a gorgeous sunset, the whole western particles which floated invisibly in the heavens glowing with roseate hues, the cubic inch of mixed air above the table. observer sees the colors melting away beBut, as there are 40 cubic inches of mixed fore his eyes and becoming transformed air in the flask and syringe, the number of into different hues. The clouds are of

different sizes and of all shapes. Some which reflect light, ceases to be illuminated, float virgin-like in silver folds, others voy- the depth and fulness of the blue most age in golden groups ; some are embroid. intensely increase.

This effect is proered with burning crimson, others are like duced by the very fine particles of dust in “ islands all lovely in an emerald sea." the sky overhead being unable to scatter And when the flood of rosy light, as it any colors unless those of short wavedeepens into bright crimson, brings out lengths at the violet end of the spectrum. into bold relief the circlet of flaming Thus we see, above, blue in its intensity mountain peaks, it is like a gorgeons trans- without any of the red colors. When, formation scene. Stranger still, when the however, the observer brings his eyes sun sinks below the horizon, and a dull down in any direction except the west, ashen gray has possessed the western he will see the blue mellowing into blueheavens, what occasions the hectic flush on green, green, and then rose-color. And the eastern horizon ! Gradually the clouds some of the most beautiful and delicate are tinged with light red from the eastern rose tints are formed by the air cooling, horizon all over the zenith ; whence comes and depositing its moisture on the parthe coloring!

ticles of dust, increasing the size of the It is a strange coincidence that these re- particles till they are sufficiently large to markably fine sunsets have been since the stop and spread the red rays, when the tremendous eruptions at Krakatoa, in the sky glows with a strange Aurora-like light. Straits of Sunda. Along with the lava The dust theory of the splendor of sunset eruption there was ejected an enormous coloring is strengthened by the often gloquantity of fine dust. The decks of ves- rious afterglows. The fiercely brilliant sels, hundreds of miles away, were covered streaks of red have disappeared; over the with it. Mr. Verbreek computed that no mountain ridge a flush of orange hovers, less than 70,000 cubic yards of dust and softens the approaching blue. The actually fell round the volcano. This will western hills, that once stood out bronzed give an idea of the enormous quantity of against the glare of light, are sombredust still floating in the atmosphere, and hued. But suddenly, as by a fairy's drifting all over the world. In the upper wand, the roseate flush of beauty rises in atmosphere, too, there must always be the east, and stretches its beautiful tints dust, for without the dust no clouds could all over the sky. As the sun sinks, but be formed to shield us from the sun's before it ceases to shine on our atmosscorching rays; and of cosmic dust there phere, the temperature of the air begins must be a considerable quantity in the air, to fall, and its cooling is accompanied by produced by the waste from the millions an increase in the size of the particles of meteors that daily fall into it. Mr. floating in it by the condensation of the Aitken has ably shown that the brilliancy water-vapor. The particles to the east lose and variety of the coloring are due to the the sun first, and are thus first cooled. suspended dust in the atmosphere. Accordingly, the rays in that direction

Observers of the gorgeous sunsets and are best sifted by the larger water-clad afterglows have been most particularly particles of dust, and the roseate coloring struck with the immense wealth of the va- is there more distinct than in the north rious shades and tints of red. Now, if the and south. As the sun sinks further, the glowing colors are due to the presence of particles overhead become cooler, and atdust in the air, there must be somewhere tract the water-vapor ; thus they increase a display of the colors complementary to in size, and thereby reflect the red rays. the reds, because the dust acts by a selec- Here the red hues, at first visible in the tive dispersion of the colors. The small east, slowly rise, pass overhead, and dedust-particles arrest the direct course of scend in the west to form the charming the rays of light and reflect them in all afterglow. Sometimes a flood of glory directions ; but they principally reflect the will roll once more along the summits of rays of the violet end of the spectrum, the hills, entrancing the attention of the while the red rays pass on almost un artistic spectator. checked. Overhead deep blue reigns in All examinations of the volcanic dust awe-inspiring glory. As the sun passes lately collected from the atmosphere show below the horizon, and the lower stratum that a great quantity of it is composed of of air, with its larger particles of dust small glassy crystals. An abundance of

these would quite account for the pecu- coloring in sunrise or sunset than the liarity in the visibility of the first glow; smaller and fewer particles on the mounand the evidence seems to indicate that the tain-top. quantity of such crystals is sufficient to It is now admitted that the inherent produce the result. When these are fully hue of water is blueness. Even distilled illuminated, they become in turn a source water has been proved to be almost exof illumination, and reflect their reddish actly of the same tint as a solution of light all around. In winter sunsets, the Prussian blue. This is corroborated by water-clad dust-particles become frozen, the fact that the purer the water is in and the peculiarly brilliant crimson is nature, the bluer is the hue. But though seen, coloring the dead beech-leaves and the selective absorption of the water dered sandstone houses, and making them termines its blueness, it is the dust-particles appear to be painted with vermilion. suspended in it which determine its brill

If, then, there were no fine dust-par- iancy. If the water of the Mediterranean ticles in the upper strata of the atmosphere, be taken from different places and examthe sunset effect would be paler ; if there ined by means of a concentrated beam of were no large particles in the lower strata, light, it is seen to hold in suspension millthe beautiful sunset effects would cease. ions of dust-particles of different kinds. In fact, if our atmosphere were perfectly To this fine dust it owes its beautiful, brillvoid of dust-particles, the sun's light iant, and varied coloring. Where there would simply pass through without being are few particles there is little light reseen, and soon after the sun dipped below flected, and the color of the water is deep the horizon total darkness would ensue. blue ; but where there are many particles The length of our twilight, therefore, de more light is reflected, and the color is pends on the amount of dust in one form chalky blue-green. Along its shores the or another in our atmosphere. Not only, Mediterranean washes the rocks and rnbs then, would a dustless atmosphere have no off the minute solid particles, which make clouds, but there would be no charming the water beautifully brilliant. sunsets, and no thought-inspiring twilights. That this is the case can be illustrated.

There is a generally prevalent fallacy If a dark metal vessel be filled with a that the coloring at sunrise or sunset is weak solution of Prussian blue, the water much finer when seen from the summit of will appear quite dark and void of color. a mountain than from a valley. To this But if some fine white powder be thrown matter Mr. Aitken bas been giving some into the vessel, the water at once becomes attention, and his observations point the of a brilliant blue color; if more powder very opposite way, corroborative of his be added, the brilliancy increases. This dust-theory. From the summit of the accounts for the changes of depth and Rigi Kulm in Switzerland he saw several brilliancy of color in the several shores of sunsets, but was disappointed with the the Mediterranean. In Lake Como, where flatness and weakness of the coloring ; there is an entire absence of white dustwhereas in the valley, on the same even- particles, the water is of a deep blue color, ings, careful observers were enchanted with but void of brilliancy ; but, where the the gorgeous display. The lower dusty Lake enters the river Adda, the increase humid air was the chief source of the of the current rubs down fine reflecting color in the sunset effects. His opinion particles from the rocks ; in consequence, is strengthened by the fact that when from there the water is of a finer blue. When the summit he saw large cumulous clouds, the dust-particles carried down by the the near ones were always snowy white, Rhone spread out into the centre of the while it was only the distant ones that Lake of Geneva, the color assumes the were tarnished yellow, showing that the deeper blue, rivalling in brilliancy any light came to these clouds unchanged, and water in the world. it was only the air between the far-distant The phenomenon called a haze puzzled clouds and his eye that tarnished them investigators until Mr. Aitken explained it yellow. On the mountain-top it required on the principle of the condensing power a great distance to give even a slight col- of dust-particles. Haze is only an arrested oring The larger and more numerous form of condensation of water-vapor. If dust-particles in the air of the valley are, one half of a dusty pane of glass be cleaned therefore, productive of more brilliant in cold weather, the clean part will remain

« הקודםהמשך »