470 Memoir on Sulphuric Ether. 
and consequently is of an unknown, but re- 
duced density.. The opposite error is liable 
to be induced, by the frequent use of the in- 
strument. By the motion of the mercury, 
the small remains of ether mechanically 
mixed with it rises to the top, and a visible 
stratum of ether is thereby exposed to the 
air. In this case an addition of force is 
given to the air; but as the quantity of this 
force is known for any temperature, it may 
be allowed for accordingly.—I prefer, how- 
ever, sealing the tube when well dried and 
the air of atmospheric density at the time; 
and if the ether appear to rise to the surface 
afterwards, the correction must be applied. 
In order to have a complete check upon this 
instrument, it should be adapted so as to be ap- 
plicable at some temperature (as 140° ), where 
the force is known by other direct means. 
The error, if any, will thus be shewn, and 
may be calculated for other temperatures. 
I have lately made, for the first time, va- 
rious experiments on the force of steam from 
water, in temperatures from 212° to 300°; 
the results which convince me that the theo- 
retic forces which I gave in the 5th Vol. of 
the Memoirs, as also, those subsequently in 
my Chemistry, are both erroneous ; the for- 
mer being about as much too small as the lat- 
