Dust

Dust, earth or other matter reduced to fine dry and powdery particles; the word is Teutonic and appears in such various forms as the Dutch duist, Danish dyst, for the dust of flour or meal, and in the older forms donst; the modern German Dunst, vapour, probably preserves the original form and meaning, that of something which can be blown about by the wind.

Atmospheric Dust.—The presence of dust in the atmosphere has probably been known from the earliest ages, as prehistoric man must have had plenty of opportunities of noticing it lighting up the paths of sunbeams that penetrated his dark caves, yet it is only of recent years that it has become the subject of scientific observation. Formerly it was considered as simply matter in the wrong place, the presence of which had to be tolerated, but was supposed to serve no useful purpose in nature. It was not till the year 1880 that atmospheric dust came under scientific investigation, when it soon became evident that it played a most important part in nature, and that instead of being a nuisance to be got rid of, it added much to the comforts and pleasures of life.

The atmosphere is composed of a number of gases which have a nearly constant proportion to each other, and of varying proportions of water vapour. This vapour, constantly rising from land and sea, mixes with the gases in the atmosphere and so long as it remains vapour is invisible, but when it becomes cooled by the actual processes in nature the vapour tends to condense to the liquid condition and form cloud particles. Before 1880 it had always been assumed that when this condensation took place, the vapour molecules simply combined with each other to form the little globules of water, but J. Aitken showed that vapour molecules in the atmosphere do not combine with each other, that before condensation can take place there must be some solid or liquid nucleus on which the vapour molecules can combine, and that the dust in the atmosphere forms the nuclei on which the water-vapour molecules condense. Every cloud particle being grown round a dust nucleus thus has a dust particle in it. The presence of dust in the atmosphere allows the condensation of the vapour to take place whenever the air is cooled to the saturation point, and if there were no dust present the condensation would not take place till the air was cooled far below that point, and become highly supersaturated; and when it did take place the condensation would be violent and result in heavy rain-drops without the formation of what we know as cloud. This might be in some ways an advantage, but living in such supersaturated air would have many disadvantages. The supersaturated air having no dust to condense on would condense on our clothes, the inside and outside walls of our dwellings, and on every solid and liquid surface with which it came in contact.

Many of the dust particles in the atmosphere which form the nuclei of condensation are extremely minute, so small as to be beyond the powers of the microscope, and at first sight it might appear to be impossible to get any reliable information as to their numbers. But Aitken, having shown that water vapour must have a nucleus to condense on, saw that this placed in our hands the means of counting the dust particles in our atmosphere, and in 1888 showed how it could be done. As water vapour in the air condenses on the dust particles present and forms cloud particles, he showed that all that would be necessary would be to cause the dust particles to become centres of condensation, when they would be so increased in size as to come within the range of an ordinary magnifying lens, and that by counting the cloud particles it would be possible to determine the number of dust particles. To carry out this idea the air under examination was placed in an air-tight receiver and saturated with water vapour. It was then expanded by an air-pump, and in this way cooled and condensation produced. The cloud particles so formed were allowed to fall on a micrometer and their number counted by the aid of an ordinary short-focussed lens. Certain precautions are necessary in carrying out this process. There must not be more than 500 particles per cubic centimetre of air, or all the particles will not form nuclei, and will not therefore be thrown down as cloud particles. When the number in the air tested exceeds that figure, the dusty air must be mixed with such a quantity of dustless air as will reduce the number below 500 per c.c., and the correct number in the air tested is obtained by allowing for the proportion of dustless air to dusty air, and for the expansion necessary for cooling.

Thousands of tests of the atmospheric dust have been made with this instrument at many places over the world, and in no part of it has dustless air been found; indeed it is very rare to find air with less than 100 particles per c.c., whilst in most country places the numbers rise to thousands, and in cities such as London and Paris the number may be as high as 100,000 to 150,000 per c.c.

The sources of dust particles in the atmosphere are numerous. In nature volcanoes supply a large quantity, and the meteoric matter constantly falling towards the earth and becoming dissipated by the intense heat produced by the friction of the atmosphere keep up a constant supply. Large quantities of dust are also raised from the surface of the earth by strong winds, from dusty roads and dry soil, and there is good reason for supposing that large quantities of sand are carried from the deserts by the wind and transported great distances, the sand, for instance, from the desert of Africa being carried to Europe. It is, however, to artificial causes that most of the dust is due. The burning of coal is the principal source of these, not only when the coal is burned with the production of smoke, but also when smokeless, and even when the coal is first converted into gas and burned in the most perfect forms of combustion. It results from this that while in the air over the uninhabited parts of the earth and over the ocean the number of particles is small, being principally produced by natural causes or carried from distant lands, they are much more numerous in inhabited areas, especially in those where much coal is burned. It is evident that if there were not some purifying process in nature there would be a tendency for the dust particles to increase in numbers, because though some dust particles may fall out of the air, many of them are so small they have but little tendency to settle, but by becoming centres of cloud particles they are carried downwards to the earth, and, further, these when showering down as rain tend to wash the others out of the atmosphere. We may therefore look on all uninhabited areas of the earth as purifying areas, and their purifying power seems to depend partly on their extent, but principally on their rainfall. The following table illustrates the purifying effect of some of these areas obtained from the results of hundreds of observations. The areas referred to are: (1) Mediterranean Sea, the observations being made on the south coast of France on the air blowing inshore; (2) the Alps, the observations being made on the Rigi Kulm; (3) the Highlands of Scotland, the observations being made at various places; and (4) the Atlantic Ocean, the observations being made on the west coast of Scotland, when the wind blew from the ocean.

	Mediterranean.	Alps.	Highlands.	Atlantic.
Mean of lowest	891	381	141	72
Mean of number	1611	892	552	338

These numbers are all low for atmospheric dust, much lower than in air from inhabited areas. On the Rigi Kulm, for instance, the number was sometimes over 10,000 per c.c. when the wind was from inhabited areas and the sun causing ascending currents; and at the same place as the Atlantic air was tested the numbers went up to over 5000 per c.c. when the wind blew from the inhabited areas of Scotland, though the distance to the nearest was over 60 m.

E.D. Fridlander1 made many observations on the dust of the atmosphere with the same instrument as employed by Aitken. In crossing the Atlantic he got no low numbers, always over 2000 per c.c., but in the Gulf of St Lawrence he got a reading as low as 280 per c.c. In crossing the Pacific the lowest obtained was 245, in the Indian Ocean 243, in the Arabian Sea 280, in the Red Sea 383, and in the Mediterranean 875 per c.c. He has also made observations in Switzerland. The lowest number obtained by him was in the air at the top of the Bieshorn, 13,600 ft. above sea-level, where the number was as low as 157 per c.c. Professor G. Melander2 of Helsingfors studied the dust in the atmosphere. His observations were made in Switzerland, Biskra in the Sahara, Finland, the borders of Russia, and in Norway; but in none of these places were low numbers observed. The minimum numbers were over 300 per c.c., while maximum numbers in some cases went high.

Aitken when observing on the Rigi Kulm noticed during some conditions of weather that there was a daily variation in the number of particles, a maximum near the hottest part of the day and a minimum in the morning, and attributed the rise in the numbers to the impure air of the valleys rising on the sun-heated slopes of the mountain or driven up by the wind. A. Rankin, at the Ben Nevis observatory, also observed this daily variation, and his observations also indicate a yearly variation at that station, the numbers being highest in March, April and May. This may possibly be due to small rainfall in these months, but more probably to the fact that south-easterly winds blow more frequently during these months on Ben Nevis than at any other season, and these winds bring the impure air from the more densely inhabited parts of the country.

Without atmospheric dust not only would we not have the glorious cloud scenery we at present enjoy, but we should have no haze in the atmosphere, none of the atmospheric effects that delight the artist. The white haze, the blue haze, the tender sunset glows of red, orange and yellow, would all be absent, and the moment the sun dipped below the horizon the earth would be in darkness; no twilight, no after-glows, such as those given some years ago by the volcanic dust from Krakatoa; none of the poetry of eventide. Why, it may be asked, is this so? Simply because all these are due to matter suspended in the air, to dust. Water has no such effects as long as it is a vapour, and if it condensed without the presence of dust, the particles would be far too few to give any appreciable effect and too heavy to remain in suspension.

Turning now to the investigations on this point, Aitken has shown that there is no evidence to indicate that water vapour has any hazing effect, and shows that the haze is entirely due to dust, the density of the haze increasing with the increase in the number of dust particles in the air, and also with the relative humidity; but the humidity does not act as vapour, but by condensing on the dust and increasing the size of the particles, as it is not the amount of vapour present but the degree of saturation that affects the result; the more saturated the air, the more vapour is condensed on the particles, they so become larger and their hazing effect increased.

The relation of haze or transparency of the air to the number of dust particles was observed on five visits to the Rigi Kulm. The visibility of Hochgerrach, a mountain 70 m. distant from the Rigi, was used for estimating the amount of haze when the air was clear. During the visits this mountain was visible thirteen times, and it was never seen except when the number of particles was low. On eight occasions the mountain was only one-half to one-fifth hazed, and on these days the number of particles was as low as from 326 to 850 per c.c. It was seen five times when the number was from 950 to 2000 per c.c., but the mountain on these occasions was only just visible, and it was never seen when the number was a little over 2000 per c.c.

It has been pointed out that the relative humidity has an effect on the dust by increasing the size of the particles and so increasing the haze. It was therefore necessary in working out the dust and haze observations made at the different places to arrange all the observations in tables according to the wet-bulb depressions at the time. All the observations taken when the wet-bulb depression was between 2° and 4° were put in one table, all those when it was between 4° and 7° in another, and all those when it was over 7° in a third. It should be here noted that when the dust particles were counted and the wet and dry bulb observations taken, an estimate of the amount of haze was also made. This was done by estimating the amount of haze on a mountain at a known distance. Suppose the mountain to be 25 m. distant, and at the time to be one-half hazed, then the limit of visibility of the mountain under the conditions would be 50 m., and that was taken as the number representing the transparency of the atmosphere at the time. In the tables above referred to along with the number of particles was entered the limit of visibility at the time; when this was done it was at once seen that as the number of particles increased the limit of visibility decreased, as will be seen from the following short table of the Rigi Kulm observations when the wet-bulb depression was between 2° and 4°.

Date.	Lowest Number.	Highest Number.	Mean Number.	Limit of Visibility in Miles.	C.
19th May 1891	428	690	559	150	83,850	}Mean 75,176.
22nd May 1889	434	850	642	100	64,200
16th May 1893	1225	2600	1912	40	77,480

When the number of particles is multiplied by the limit of visibility in the tables a fairly constant number C. is obtained; see preceding table. All the observations taken at the different places were treated in a similar manner and the means of all the observations at the different humidities were obtained, and the following table gives the mean values of C. at the different wet-bulb depressions of all the observations made at the different places.

Wet-bulb depression	2° to 4°	4° to 7°	7° and over
Mean values of C.	76,058	105,545	141,148

From the above table it will be seen that as the dryness of the air increased it required a larger number of particles to produce a complete haze, nearly double the number being required when the wet-bulb depression was over 7° than when it was only from 2° to 4°. To find the number of particles required to produce a complete haze, that is, to render a mountain just invisible, all that is necessary is to multiply the above constant C. by 160,930, the number of centimetres in a mile, when this is done with the observations made in the West Highlands we get the numbers given in the following table:—

Wet-bulb depression.	Number of Particles to produce a complete haze.
2° to 4°	12,500,000,000
4° to 7°	17,100,000,000
7° to 10°	22,600,000,000

The above table gives the number of particles of atmospheric dust in a column of air having a section of one centimetre square, at the different humidities, required to produce a complete haze, that is, to make a distant object invisible, and is of course quite independent of the length of the column.

In making these dust and transparency observations three things were noted: 1st, the number of particles; 2nd, the humidity; and 3rd, the limit of visibility. From the results above given, it is evident that if we now know any two of these we can calculate the third. Suppose we know the limit of visibility and the humidity, then the number of particles can be calculated by the aid of the above tables.

To show the hazing effects of dust it is not, however, necessary to use a dust counter. Aitken for some years made observations on the haze in the air at Falkirk by simply noting the direction of the wind, the wet-bulb depression at the time, and the transparency of the air. Falkirk is favourably situated for such observations owing to the peculiar distribution of the population surrounding it. The whole area from west, north-west to north, is very thinly populated, while in all other directions it is densely populated. It was found that the air from the thinly inhabited parts, that is, the north-west quadrant, was nine times clearer than the air from other directions with the same wet-bulb depression, and that the density of the haze was directly proportional to the density of the population of the area from which the wind blew. These observations also showed that the transparency of the air increases with the dryness, being 3.7 times clearer when the wet-bulb depression is 8° than when it is only 2°, and that the air coming from the densely inhabited parts is about 10 times more hazed than if there were no inhabitants in the country.

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