48 



ATMOSPHERIC DUST; 



Tuhle XXXI. Mechanical Composition of Dust Collected from Surfaces elevated some Distance 

 above the Ground. (Unless otherwise stated the collecting was done at Rock Island, III. ) 



Length of 

 diameter in 



Shaken from the 



barli of an oali 



tree. 



128 



Taken In rain 



water f r. the roof 



of a house. 



From rain water 



from the bark of 



aa oak tree. 



From 



the trunk of an 



oak tree. 



Taken in rain 



water fr. the roof 



of a house. 



lG-8 

 8-4 

 4-2 

 2-1 



1 1 



1 1 



TTT TS 



1 



ttV 



1 ^ 1_ 



tr. 



tr. 



tr. 



2.9 

 38.5 

 41.3 

 19.2 



I 



i 



leaves against each other. When collected in the early 

 part of the summer this dust is therefore found to be 

 coarser than it is later on, owing to the more frequent 

 removal of the coarser particles and the more persistent 

 adhering of the finer. In some dust which was washed 

 from the leaves of some oak trees in the months of May 

 and June in 1895, there was about twenty per cent of 

 fine dust (Nos. 136, 137), while in two samples taken in 

 August and September, there was a. little over thirty per 

 cent of the same ingredient (Nos. 138, 139), and in 

 another sample, which was washed from leaves remaining 

 on some trees in February, the fine dust was the maxi- 

 mum ingredient making nearh' forty per cent of the wdiole 

 sample. Four of these analyses are of dust taken on the 

 bai'k of some trees, and two are of dust coming with rain 



