On the Expansion of Elastic Fluids by Heat. 597 
the aqueous vapour may be drawn out of the 
tube; this is essential if we operate in temper- 
atures lower than that of the atmosphere, other- 
wise not. For want of this: attention, Col. 
Roi, in his valuable paper in the Philos. Trans. 
vol. 67, has been led into some erroneous con-. 
clusions.—A small column, of dry mercury is 
then let down toa proper point in the mano- 
meter, and it is ready for experiment with 
common air, 
It requires some address to fill the manometer 
with any other gas,—I succeeded best as follows : 
filled the tube with dry mercury; then pushed 
down a wire with thread,. so that when the wire 
was got to the end of the tube, a thick covering 
of thread just entered the open end, and held 
the mercury like a cork, so that the tube could 
be inverted without losing the contents; then 
having a glass funnel with a perforated cork 
over the water apparatus, containing the gas, 
I slipped the manometer through the hole in the 
cork, and putting my hand into the water under 
the funnel, drew the wire out of the manometer, 
and with it the mercury; upon which the gas 
entered the manometer. For carbonic acid gas, 
I opened the sealed end of the manometer, 
drew it out to a capillary bore, and forced a 
stream of the gas through the tube; then putting 
my finger on the other end, sealed it again bya 
