322 
SELECTED ARTICLES. 
smallest found in commerce, of which about six will weigh 
one grain. 
A quantity carefully determined by previous weighing, is 
to be used in each experiment, and is to be very nearly equal 
to one-fifth the weight of water which the matras will con- 
tain. The weight of water for moistening the shot should be 
one-seventeenth part of that of the metal when dry; a quantity 
either greater or less than this retards the operation. An ex- 
cess of water has the additional inconvenience of forming a 
scum, which interferes with the accurate determination of the 
volume of gas, left after absorption. Hence the water which 
moistens the shot is less than li per cent, of the volume of 
air under examination. The matras charged with moistened 
shot, and with the mouth open, is exposed for two or three 
hours in the open air, or the air is renewed with a bel- 
lows with a recurved nozzle. Having observed the tem- 
perature and pressure of the air, the matras is closed by means 
of the keys already mentioned. In the analysis of other 
gases than atmospheric air, a retort, closed by a stopcock, is 
substituted for the matras already described, and the shot is 
kept in the body of the instrument. Having exhausted this 
instrument, the gas to be tried is admitted. In some cases, 
the air-pump may be dispensed with by filling up the vessel 
with water after having put in the shot. The water is then 
displaced by admitting the mixed gases from the pneumatic 
cistern. The shot is drained by inclining the retort, still 
held over the cistern. The retort is now plugged with a me- 
tallic screw to bring the shot in contact with the mixed gases, 
and the remaining gas is transferred to a graduated tube in 
order to be measured. Before transferring, however, the re- 
tort neck is plunged under water, of a lower temperature than 
that at which it was filled. A momentary condensation en- 
sues, intended to prevent the possibility of any escaping in 
case no absorption had taken place. 
In experimenting on atmospheric air, the analysis of which 
admits of greater exactness than that of other gases, the grains 
of moistened lead which have hitherto remained unaffected 
