ON THE USE OF LEAD IN EUDIOMETRY. 323 
by the air from being carefully preserved from agitation, are 
now to be violently shaken entirely in the bulb of the matras, 
so as to avoid tarnishing the neck. The bulb becomes coated 
with a yellow pigment, which by three hours of agitation 
changes to a brownish gray tint, due to a mixture of yellow 
oxide, with very fine particles of lead, indicating clearly that 
all the oxygen gas has disappeared. This process affords very 
pure nitrogen gas. Nitrous gas produces not the slightest 
diminution of its volume. The method of closing the matras 
with a screwed stopper is so effectual that it may be left, and 
the agitation of the shot suspended for an indefinite period. 
Having weighed the matras by means of a balance sensible 
to one or two-tenths of a grain, it is opened while inverted 
under water, the stopper is replaced by an open stopcock, and 
while inverted, it is fixed in a ring which by sliding up and 
down may allow of adjusting the height so as to bring the 
water to the same level within and without the neck. The 
temperature and pressure of the air are again observed, and 
the stopcock is then closed, which should be so easily turned 
as not to require placing the hands on the matras. The dif- 
ference in weight of the vessel when partly filled with water, 
after the absorption, and of the same when entirely filled with 
water, shows how much gas remains after the absorption is 
complete. By a similar process, the volume of air before 
absorption is measured, making due allowance in both cases 
for the approximate weight of air or gas displaced by the 
water. 
If the neck of the matras were correctly graduated, we 
might determine by inspection the amount of absorption; but 
this mode of estimating quantities is too uncertain, and the 
graduation on a large irregular tube, generally too imperfect 
to give even a tolerable approximation to the degree of ex- 
actness obtained by weighing. 
Though it is not proposed to substitute this process by the 
oxidation of lead, in ordinary analysis, for that of Volta — so 
prompt and convenient in execution, and so indispensable 
in various investigations — yet it will be found that the oxi- 
