THE Wairarapa Age MORNING DAILY. WEDNESDAY, MARCH 27, 1912. "DRY AS DUST."

Wairarapa Age, Volume XXXII, Issue 10593, 27 March 1912, Page 4

THE Wairarapa Age MORNING DAILY. WEDNESDAY, MARCH 27, 1912. "DRY AS DUST."

If the resvit of -scientiiia research in recent years' .stands for anything, the term "dry as dust" should come into universal disrepute, for it has been demonstrated that atmospheric dust plays.an important part in nature, and that, instead'of-being a nuisance to be got rid of, it adds 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 pro'oesses 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, tile vapour molecules simply combined with each other to form the little globules ■of water, but J. Aitken showed that vapour molecules, in the atmosphere d'o not combine with each other, that before condensation can take place there must be some isolid or liquid nucleus on wihioh the vapour molecule scan combine, and that the dust in the atmosphere forms the nuclei on which tho water-vapour molecules

condense. Every cloud particle being grown round a dust nucleus thus- lias I 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 '" the air wan cooled far below that point, and hecome highly super-saturated; and when it did take place the condensation avouM he violent, and result in heavy rain-drops without the formation of what we know as cloud. This miglvt he in some ways an advantage, but living in .such super-saturated air would havo.'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, iso small as to he beyond the power of the microscope, and at "firstsight it might appear to he impossible to get any reliable information as to their numbers. But Aitken, having shown that water vapour must have a nucleus to con.demse on, saw tbait this placed in our hands the means of counting tho dust particles in our atmosphere, and in 18&S showed how it could be done. As water vapour in the air condense® on the dust particles present and foiims cloud particles, he shoAved 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 re- ; oeiver 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 micrometer and their number counted by the aid of an ordinary .short-ifocussed lens. Certain precautions are necessar. Yin carrying out this process. There must not be more '.than 500 particles -per cubic centimer : or all the particles ■ will not; foi-mi nuclei, and'will hot therefore be thrown ;down : as cloud pari--oles. When, the number in.the attested, exceeds: that figure,'. the dury air must' ho mixed with such a quantity of dust-less air as will reduce the number helow 500 per e.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 Pooling, v"'