METALS IN THE ATM(>SPHP:RK. 237 



wind Mild the Iniiiiidity of the aliiiospluTc, also with (he season of the 

 ycai' and the state of the ^uii. 



In Paris between ti' and 1) uic;. of dust rtre collected on a square 

 meter of surface in twenty-four houi's. Takinir 4 nig. as the mean, 

 this coi'responds to a daily deposition on a sni'face eqnal to that of 

 the Champs de Mars (about nOO.OOO m-) of -2 kilos (4.4 pounds) 

 of corpuscles. An estimate made by forcing air through water and 

 evaporating shows a weight of (> to 2^^ mg. to a cubic meter of air. 

 Taking in this case (> mg. as the mean and considering a sheet of air, 

 say, T) m. in tiiickness, there woidd be almost IT) kilos (33 pounds) of 

 dust in an area e(|ual in extent to the Champs de Mars and a weight 

 of several hundred kilos in air overhanging Paris. In the fields 

 the (pumtity of dust collected is considerably smaller. It is hardly 

 necessary to say that this sediment does not remain long in the place 

 where it falls, but is (juickly carried otf again by the wind. However, 

 the figui'es given will indicate in a slight measure the importance and 

 extent of this aerial transj)oi"tation of solid matter. 



High buildings act as veritable dust traps. For instance, in a 

 tower of Notre Dame Cathedral which no one had entered for sev- 

 eral years, the wind, passing through the narrow windows (iO m. from 

 the ground, had deposited a bed at least a millimeter in thickness of 

 fine grayish dust. Analysis showed that this was of the same com- 

 position as the atmospheric dust, i. e., about 32 per cent organic mat- 

 ter and ()7 per cent of cinders. Of this inorganic matter, 1) per cent 

 was s()lul)le in water, 24 ])er cent in hydrochloric acid, and the remain- 

 ing 34 per cent consisted of a residue essentially silicate. Various 

 analyses of dust accumulated in uninhabited jjortions of lofty struc- 

 tures showed very similar results; the dimensions of the particles 

 were invariably compassed between 0.01 and 0.001 nnn., and the 

 cinders always rejn-esented about 75 per cent of all the matter 

 collected. 



Anothei- method of investigating the solid bodies in the atmosphere 

 is by making an analysis of meteoric water. Rain is always chai'ged 

 with a sediment collected in the air, which may be extracted by 

 filtering and evaj)oi"atiou. It can be advantageously collected in a 

 receptacle made of a series of j^orcelain plates, built up on three 

 sides only and ai'ranged on a frame like tiles on a roof. If this 

 apparatus be of sutticient size considerable quantities of water will 

 glide over the phujues and through a funnel into a lai'ge flask. Porce- 

 lain is unu'h better than glass for this ])in'pose, as the latter is likely 

 to be attacked by the carbonic acid and annnonia in standing rain 

 Avater. 



The weight of the residue extracted from a liter of water varies 

 cf)nsiderably. At the observatory of St. Marie du Mont (Manche) 

 the sediment obtained by evaporation of several liters of rain water 



