236 METALS IN THE ATMOSPHERE. 



In this residue may be distinguished corpuscles of organic matter, 

 with Avhich we are not now concerned, and also mineral substances, 

 which we will examine more particularly. Tiny as these bodies are, 

 it is possible to measure them Avith a finely graduated micrometer, and 

 it has been found that in diameter they ordinarily measure between 

 one one-hundredth and one one-thousandth of a millimeter. 



It is also possible to obtain an approximate idea of the quantity of 

 these dust particles by drawing with an air pump a determined amount 

 of air, bubble by bubble, through a tube containing a little pure w^ater 

 and then through a wad of nitrated cotton. The particles which have 

 been retained by the water are secured by evaporation and united 

 with those obtained by the dissolution in ether of the guncotton. 

 By this method there have been foinid at Paris, during normal 

 atmospheric conditions, from (> to 8 mg. of dust to the cubic meter 

 of air; after a day's rainfall, G mg. ; after a drought of eight days, 

 28 mg. Naturally the quantity in country air is much smaller. These 

 figures represent the total Aveight of all solid particles, mineral and 

 organic; if the latter be eliminated by calcination in a current of air 

 the cinders re})resenting mineral matter will be found to vary from 

 C6 to 75 per cent of the whole weight. In the residue are found 

 cinders of salts soluble in water, of matter soluble in hydrochloric 

 acid, and of substances which can not be dissolved either in water or 

 in acid. 



The particles lioating in the air are held there only by atmospheric 

 agitation, the nu)st minute being held longest in suspense. It may 

 well l)e asked how bodies of this kind, so nnich heavier than the air, 

 can be held in the atmosphere. Circulation will show that grains of 

 mineral as small as 0.01 nun. in diameter can nevertheless fall with 

 considerable rai)idity, 0.()() m. a second in the case of a gram of silica. 

 It can easily be seen that a sphere of t2..5 density. 1 m. in diam- 

 eter, would fall at a speed of 220 m. a second, if the fall were 

 uniform and through air of ordinary density. The theoretical 

 A^docity of a c()ri)uscle of d dimensions Avould therefore equal 4220d. 

 But in reality this is greatly modified in bodies of minute dimensions 

 by the agitation and continual movement of the air. which fact 

 accounts for the suspension of the atmospheric i>articles. They do, 

 however, fall grachially, and are contiuuaily forming on the earth's 

 surface a sediment that can easily be collected l)y stretching on a 

 frame a sheet of i)aper treated wilh gehitin and placing the con- 

 trivance on an isolated roof 10 to 1.') m. from the ground. Or a dust 

 table, about a nietei- scpiare, lined with thin sheet tin and turning on 

 an axis so as always to face the wind, can be used with ecjual success. 

 The wind passing ()\('r itN surface constantly lets fall a portioii of 

 the dust it carries, aiul this deposit is afterwards collected Avith a flat 

 hair biMish. The (luantitv of dust varies with the velocity of the 



