MKTALS IN TIIK ATMOSPHERE. 287 



wind iiii<! (he lnmii(li(y of (lie iidiiosplici-c. also with the season of i\\v 

 year and llic stale of (he snn. 



Ill Paris hetwH'eii '2 and '.) \\\ix. of (hist ai'c eonectod on a s(inar(> 

 meter of snrfaee in t\\ enty-t'onr honi's. 'ralvin<>; 4 nig. as the mean, 

 this ooiTcsponds to a (hiily deposition on a snrface equal to that of 

 the Champs de Mars (about 500,000 m-) of 2 kilos (4.4 pounds) 

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

 evaporating shows a weiglit of (> to 2?> mg. to a cubic meter of air. 

 'leaking in this case mg. as the mean and considering a sheet of air, 

 say. ."> m. in thickness, there wouhl be almost 15 kilos (33 pounds) of 

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

 of several hnndrcMl kihis in air overhanging Paris. In the fields 

 (he (juantity of dust collected is considerably smaller. It is hardly 

 n^'cessary to say that this sediment does not remain long in the j)lace 

 where it fal's, l)nt is (|iiickly carried otf again by the wind. However, 

 the fignres given will indicate in a slight measure the importance and 

 extent of this aerial transportation of solid matter. 



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

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

 eral years, the w^ind, passing through the narrow windows (iO m. from 

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

 tine grayish dust. Analysis showed that this w'as of the same com- 

 position as the atmospheric dust, i. e.. about o2 per cent organic mat- 

 ter and ()T per cent of cinders. Of this inorganic matter, !) per cent 

 was solubU' in Avater, 24 per cent in hydrochloric acid, and the remain- 

 ing 34 pel' cent consisted of a residue essentially silicate. Various 

 analyses of dust accumulated in nninhal)ited jjortions of lofty struc- 

 tures showed very similar results; the dimensions of the particles 

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

 cinders always represented about 75 |)er cent of all the matter 

 collected. 



Another method of investigating the solid bodies in tlu^ atmosphere 

 is by making an analysis of meteoric w^ater. Rain is always charged 

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

 filtering and eva})orati()n. Tt can be advantageously collected in a 

 receptacle made of a series of porcelain jjlates, built u\) on three 

 sides only and arranged on a frame like tiles on a roof. If this 

 a[)paratus be of sufficient size considerable quantities of water will 

 glide over the plaques and tlirongh a fnnnel into a large flask. Porce- 

 lain is nnich l)etter than glass for this pnri)ose, as the latter is likely 

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

 water. 



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

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

 the sediment obtained by evaporation of several liters of rain water 



