192 Proceedings. 



and more than two-thirds belong to the Genus Achatinella. 

 The fresh-water shells are very sparse, and for the most 

 part very small in size, numbering altogether about 25 

 species, belonging to eight genera. The species exhibited 

 by Mr. Rogers were representatives of the genera Nanina, 

 Helix, Helicina, Succinea, and Limnea. 



Mr. J. F. Allen read the following communication: 

 "During the long frost of this winter a thin layer of frozen 

 snow lay upon the ground for some weeks. This snow is 

 an excellent medium for obtaining deposits of soot and 

 other impurities out of the atmosphere. During the frost 

 there was a prevalence of easterly winds, from N.E. to 

 S.E. When I thought a thaw was imminent, I carefully 

 collected a sufficient quantity of surface snow to afford a 

 fair sample. Alexandra Park and Whitworth Park lie from 

 Manchester in a similar direction to Ditton Junction and 

 Hale Bank from Widnes — that is, easterly winds bring Man- 

 chester smoke over Whitworth and Alexandra Parks, and 

 Widnes smoke over Ditton Junction and Hale Bank. On 

 February 6th, 1895, samples were collected near Widnes : — 



1st. At a piece of open land north of the Liver works. 



2nd. From a field near Clap Gate, just beyond Ditton 

 Junction, next to Mercer's land. 



On February 7th, from Alexandra and Whitworth Parks. 



On February 19th, from a drift along a hedge-side 

 opposite Clap Gate, going in a north-westerly direction. 

 The snow in the Manchester parks had disappeared. 



The snow was allowed to melt in the glass vessels in 

 which the samples were put, and then filtered. 20,000 

 grains of snow were measured, and the quantity of dirt 

 deposited ascertained by weighing the residuum left on the 

 filter. 



1. Snow taken near Liver works, February 6th, tested 

 neutral, neither acid nor alkaline, and contained, in 20,000 

 grains, 10-05 grains of black sooty deposit. 



