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SCIENCE-GOSSIP. 



DUST. 

 By J. O. Symes, M.D. 



TPHERE are many ways in which this subject 

 might be treated, but steering clear of the 

 philosophic side of the subject, it is my purpose 

 here to treat of atmospheric dust, "the gay motes 

 that people the sunbeams," "the dust of the dusty 

 to-day, which is the earth of an earthy to-morrow." 



Properly looked at, everything is dust. The 

 original state of matter before the formation of the 

 world was probably that of dust, and we may 

 compare the process of transformation to that 

 which to-day is taking place in the lesser stars. 

 Such stars are cloud-like masses of particles 

 travelling through space with such velocity and 

 impinging against one another with such force as 

 to become red-hot. Our stars, in short, are in- 

 candescent dust clouds, fated either to become 

 molten masses cooling to new worlds, or to be 

 absorbed into existing planets ; or, failing either 

 of these, to reach our or other worlds in the form 

 of meteoric dust. Every day we are reminded 

 that we ourselves are of the same material. In a 

 couple of hours the process of cremation can 

 convert the human body into about three pounds 

 of fine ash. 



Coming now to the dust of the atmosphere : there 

 is no atmosphere free from dust. It is to be 

 detected in the air of the highest mountains and in 

 that taken from the most distant part of the widest 

 ocean. It may vary in quality, it may vary in 

 quantity, but it is universally present. 



In an age in which savants have learned to 

 count those vibrations of ether which give rise to 

 light and sound, it is not to be marvelled at that 

 they have also succeeded in numbering the motes 

 in the sunbeam. Thus, it has been calculated that 

 in the open country each cubic inch of air contains 

 on an average two thousand dust particles, whilst in 

 towns this figure is increased to three millions, and 

 in inhabited rooms to thirty millions per cubic inch. 

 As might be expected, the range of variation is very 

 great ; the higher the altitude the fewer the number 

 of dust particles. The air of the country is freer 

 than that of the city, and that of the streets less 

 laden than that of inhabited rooms. 



Mr. Aitkin, to whose work upon this subject 

 we owe the greater part of our knowledge, records 

 some remarkable results, dependent probably 

 upon the direction of the wind and the presence 

 or absence of rain. On a peculiarly still, clear 

 day, a sample of air on the top of Ben Nevis con- 

 tained only thirty-four dust particles per cubic 

 inch ; whilst under other conditions of .wind and 

 weather, air from the summit of the Rigi contained 

 10,000, and in the Simplon Pass 200,000 particles. 



These few examples are sufficient to demonstrate 

 how wide the variations may be. There is a fas- 

 cination in the work of counting which threatens 

 to make it a craze, like bicycling, and only last 

 year an enthusiast made a voyage round the world 

 armed with Aitkin's pocket dust-counter. This 

 attempt to find an atmosphere free from dust 

 was a fruitless one. He reached an altitude of 

 over 13,000 feet, and it was there; and on the 

 open sea, so far from land as to preclude the 

 possibility of artificial pollution, dust could still 

 be detected. Very curious are the phenomena 

 connected with dust at sea. Off the west coast 

 of Africa it frequently falls in dense clouds 

 upon ships 500 to 1,000 miles from the coast. It 

 may lie so thickly upon the water as to give it the 

 colour and appearance of land, and the vessel's 

 track may be marked in it for miles. Such an 

 occurrence was noted by Darwin during his voyage 

 in the "Beagle." The barren coral reefs of the 

 Pacific Ocean receive their first layer of soil by 

 similar means, and with the dust come those low 

 forms of animal life whose part it is to crumble 

 the rock into a soil, and fit it for the reception of 

 air-borne seeds and spores, from which the island 

 vegetation must spring. Dust is then a normal 

 constituent of all atmospheres, and as such plays 

 a very important part in the economy of nature. 



Were there no dust there would be no mist, no 

 fog, no clouds, no rain ; for it is around these 

 microscopical portions of matter that the con- 

 densation of aqueous vapour takes place. Each 

 particle is enclosed in a covering of moisture, thus 

 producing a haze, a fog, or a cloud, according as 

 the dust is present in greater or smaller quantities. 

 Without the presence of dust, the sudden cooling 

 of an atmosphere saturated with moisture does not 

 result in the formation of mist or rain. This is 

 beautifully shown by allowing a jet of steam to 

 issue into a chamber containing air, filtered through 

 cotton-wool ; the dense white cloud, such as we 

 are accustomed to see over every engine-funnel, 

 does not result. In fact, our smoke and " clouds 

 of steam " are simply evidences of the presence of 

 dust. In this way dust more than anything else 

 determines the distinctness of our view of distant 

 objects ; the clearness or haziness of the landscape 

 depending upon the number of particles in the 

 atmosphere at the moment. 



To a certain extent, however, the intensity of 

 daylight, and indeed of artificial light, is dependent 

 on the presence of dust. Without it there would 

 be little or no scattering of light, as is proved by 

 the fact that a ray of light passing through a 



