ON THE USE OF LEAD IN EUDIOMETRY. 
325 
ing a residuum, rendered impure by the vapour of phospho- 
rus, by phosphuretted hydrogen, and pure hydrogen, which 
accompanies the latter. These gases are produced, not by the 
phosphorus, but by the continued contact of water with 
oxide of phosphorus. Though these inconveniences may, in 
a great measure, be remedied by washing the residuary gas 
with a solution of potash, still they do not allow of obtaining 
any greater degree of certainty, than to fix the proportion of 
oxygen in common air between 20 and 21 per cent. When- 
ever hydrogen exists in air as an impurity, the method of 
analyzing by phosphorus, even at low temperatures, cannot be 
adopted. 
The lead process is more correct than that in which hydro- 
sulphurets are employed, in proportion as the quantity of 
water required in the latter case, is so much greater. When 
not already saturated with nitrogen, this liquid will absorb 
some of that gas from the residue of the analysis; and if fully 
saturated, it is liable to part with some, and thus increase the 
quantity remaining after the operation. It is also superior to 
the same process, when applied to determine the quantity of 
oxygen in mixture with carburetted hydrogen; for the latter 
is absorbed in certain proportions by the hydrosulphuretted 
liquid, according to its temperature and the degree of its con- 
centration. The eudiometric indications of lead will possess 
a greater degree of exactness, when, instead of measuring the 
absorption of oxygen by its volume, it shall be determined by 
the weight which the metal gains by oxidation. It will only 
be necessary to dry in vacuo, and in the same vessel in which 
the oxidation was produced, the residuum — the composition 
of which is not yet accurately determined. 
Lead in contact with air and water, absorbs carbonic acid. 
The open air contains too small a proportion of this acid to 
give an indication which, at a single trial, could be distinguish- 
ed from the errors of observation. The mean of several trials 
may, however, give a more precise result. From several ex- 
periments, made in the day time, I have found as a mean, in 
100 parts of air, 21.05 per cent, of oxygen and carbonic acid. 
