218 Scientific Proceedings, Royal Dublin Society. 
from below, and pressing this against the base of the piston, as 
much mercury as it may be desired to remove is caused to fall 
down past the float. The mercury remaining, the float now 
rises into a new position in the tube. P, and V, may then be read. 
The advantage which this tube possesses over Boyle’s is, I 
think, evident. The complication of the rise of mercury in the 
upturned end of the latter being avoided, and, indeed, the opera- 
tions are evident at a glance. 
The Air Thermometer._-Drop the piston down the tube—as 
before, with float uppermost—and seal with mercury when about 
two-thirds down. Then place vertically with the lower 30 cm. in 
ice, supporting the tube by a loosely closed clip fixed to a retort- 
stand. When a few minutes in the ice add mercury till the air is 
compressed to the volume 273 millimetres. It is best to adjust a 
little indiarubber band, at this level, previously. Now lift up the 
tube and remove the ice, substituting a flask with hot water, 
carried upon a flat pattern Bunsen burner, and having a neck so 
long that it rises some 40 cm. upon the tube. Bring the water 
up to boiling, and placing a little cotton-wool in the mouth of the 
flask, to secure that the tube is for 40 cm. immersed insteam. The 
piston and mercury rapidly rise during this operation. A second 
rubber ring upon the tube is now slipped down to mark the 
stationary position finally attained by the lower surface of 
mercury. If the tube is now lifted from the flask and the length 
up to the ring measured, it will be seen to be 373 millimetres. 
Therefore we have shown that 
Vite 
Vio) 818 
In fact, there has been an expansion of z15rd of the volume at 0° 
for each degree rise of temperature. Hence, if we had been able 
to cool the gas 273° below 0°, the gas would have zero-volume 
(theoretically). Therefore 273° C. is temperature of theoretical 
zero-volume. We then draw a figure of the tube, numbering it 
on one side—273° C., 0° C., 100° C., 200° C., etc., from below up- 
wards. On the other side, at same points, we write consecutively 0, 
278, 873, 478, etc., and explain that this last scale has the obvious 
advantage of expressing at once the volume of the thermometric 
substance, e.g. the gas. So that when a change of temperature 
