266 Analysis of Atmospheric Air, by means of Nitric Oxide. 
On each side of the cylinder, there is a strip of wood, which being 
covered with white paper, is made to receive graduating lines in the 
following way. The cylinder having been filled with water, the 
lines are so applied as to indicate the changes of the level succes- 
sively produced in the surface of the water within the cylinder, by 
the successive introduction of equal volumes of air. ‘These gradua- 
tions are so proportioned, as to render the portion of the cavity com- 
prised within three of them equivalent in content, to one measure of 
the volumeter already described. In all there are nine graduations. 
In operating with this instrument, I commence by exhausting the 
air from the cylinder, and thus causing the water of the pneumatic 
cistern, over which it is situated, to rise to the fifth graduation. The 
volumeter may be filled at the same time, if the cocks between it and 
the cylinder be opened. Care must be taken to close them as soon 
as the water reaches the apex, so as to prevent the lead tube from 
being obstructed by water. The valumeter should, in the next 
place, be filled with nitric oxide gas. The apparatus thus pre- 
pared, it is only necessary to open the cocks, between the volumeter 
and the cylinder, in order to cause the nitric oxide to pass from the 
one to the other. Copious red fumes of nitrous acid immediately ap- 
pear. By means of the gum elastic bag, and recurved tube, jets of 
water are next to be thrown up into the mixture, by which the ab- 
sorption of the fumes is promoted. When these have all been ab- 
sorbed, there will appear to have been a condensation of about three 
volumes and a fifth, so that the water will have risen a little above 
the point to which it has been supposed to be raised agreeably to 
the premises. 
For the satisfaction of the spectators, the accuracy of the gradua- 
tion may be proved by allowing the contents of the volumeter in at- 
mospheric air to pass in three times, showing that the water is there- 
by depressed to 3rd, 6th, and 9th graduations. Also, by adding the 
contents of the volumeter containing three of the volumes indicated 
by the scale, to five previously introduced; thus, showing that the 
aggregate will be eight volumes, instead of less than five, as when 
three of nitric oxide are admitted to five of air. 
