236 METALS IN THE ATMOSPHERE. 



Tn this residue may be distinguished corpuscles of organic matter, 

 with wliicli 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 with a finely graduated micrometer, and 

 it has been found that in diameter they ordinarily measure between 

 one one-huiulredth and cme one-thousandth of a millimeter. 



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

 these dust particles by draw ing witli an air pump a tletermiued amount 

 of air, bubble by l)ubl)Ie. through a tube containing a little pure water 

 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 found at Paris, during normal 

 atmospheric conditions, from 6 to 8 mg. of dust to the cubic meter 

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

 23 mg. Naturally the quantity in country air is nnich smaller. These 

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

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

 the cinders representing mineral matter will be found to vary from 

 06 to 75 per cent of the whole weight. In the residue ai-c fouiul 

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

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

 in acid. 



The particles floating \n the air are held there only l)y atmospheric 

 agitation, the most minute being held longest in susi)ense. It may 

 well be asked how bodies of this kind, so much heavier than the aii\ 

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

 jnineral as small as 0.01 mm. in diameter can nevertheless fall with 

 considerable rapidity, 0.66 m. a second in the case of a gram of silica. 

 It can easily be seen that a sphere of 2.5 density. 1 m. in diam- 

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

 uniform and through air of ordinar}^ densit}'. The theoretical 

 velocity of a corpuscle of <i dimensions would therefore equal ^| 220d. 

 But in realit}^ this is greatly modified in bodies of minute dimensions 

 \>y the agitation and continual movement of the air. which fact 

 accounts for the suspension of the atmospheric particles. They do, 

 however, fall gradualh^, and are continually forming on the earth's 

 surface a sediment that can easily be collected by stretching on a 

 frame a sheet of paper treated with gelatin and placing the con- 

 trivance on an isolated roof 10 to 15 m. from the ground. Or a dust 

 table, about a nu'ter square, lined with thin sheet tin and turning on 

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

 The wind passing over its sui-i'ace constantly lets fall a portion of 

 the dust it carries, and this deposit is afterwards collected with a fiat 

 hair brush. The quantity of dusl vaiies with the V(>locity of tlie 



