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A paper was last read, ‘‘ On the foul air in the chalk and strata 
above the chalk near London,” by James Mitchell, LL.D., F.G.S. 
In the chalk, the most abundant deleterious gas is the carbonic 
acid, but it has been found to exist in greater quantity in the lower 
than in the upper portion of the formation, and in that division to 
be unequally distributed. In sinking wells, it has been noticed to 
issue with force from one stratum, whilst none has been perceived to 
be given out from the beds immediately above and below it. Dr. 
Mitchell mentions fatal effects due to its occurrence in a well near 
the race-course at Epsom, where it was met with at the depth of 
200 feet ; and in Norbury Park, near Dorking, at the depth of 400 
feet. On Bexley Heath, after sinking through 140 feet of gravel and 
sand and 30 feet of chalk, it rushed out and extinguished the can- 
dles of the workmen; and in making a well in Long Lane, Bexley 
Heath, after penetrating 124 feet of overlying deposits, and then 90 
feet of chalk, considerable inconvenience was felt from it, but 6 feet 
lower no gas was emitted. 
In chalk, sulphuretted hydrogen gas is also occasionally met with, 
and is supposed to be generated from the decomposition of water and » 
iron pyrites. 
In districts in which the chalk is covered with sand and London 
clay, carburetted hydrogen gas is sometimes emitted, but more fre- 
quently sulphuretted hydrogen gas. 
Carburetted hydrogen has seldom inflamed in wells, but in making 
the Thames tunnel it has sometimes issued in such abundance as to 
explode by the lights and scorch the workmen. 
Sulphuretted hydrogen gas is more abundant, and it has been ob- 
served almost always to proceed from a coarse black sand charged 
with oxide of iron, whether the bed be above the blue clay, within it, 
or below it. It has streamed out with great violence in the Thames 
tunnel, but has in no instance produced fatal effects. At Ash, 3 miles 
from Farnham, a well was dug in sand to the depth of 86 feet, and 
one of the workmen on descending into it was instantly suffocated. 
Fatal effects have also resulted from the accumulation of this gas in 
wells in Maiden-lane, Battle-bridge, and at Applebury-street, near 
Cheshunt. This gas is much increased, after long-continued rain, in 
consequence of the swelling of the clay driving it out of the in- 
terstices; and it is diminished after a long drought. The preva- 
lence of a north-east wind has been noticed by well-diggers to dimi- 
nish the quantity of the gas, but the effect is ascribed by Dr. Mitchell 
to the dry weather which usually accompanies the wind from that 
quarter. The author also suggests, that if wells are to be dug in 
dangerous districts, they should be undertaken when there is least 
water in the ground, or from the beginning of July to October. 
The noxious gas in the Weald of Kent and Sussex is stated to be 
sulphuretted hydrogen. 
November 20.—John Williams, Esq., Lieutenant, Royal Engi- 
neers, employed on the Trigonometrical Survey of Ireland, was elected 
a Fellow of this Society. 
