218 Scientific Proceedings, Royal Dublin Society. 
In order to increase the amount of carbonic anhydride in the air in 
the upper part of the tube'to the extent of 1 in 10,000 volumes, 2 ¢.c. of 
that gas must be added to it, and therefore 200 c.c. of ground air. 
Let us now see to what extent changes of temperature and pressure, 
both separately and together, will affect the amount of carbonic anhy- 
dride in the atmospheric air contained in the tube; and for this purpose 
let us assume that the changes occur suddenly, and that the ground air 
instantaneously diffuses into the two-metre layer of atmosphere. As to 
the effects of changes of temperature alone, each rise of 1° C. (supposing 
the initial temperature to be zero) will liberate 183 c.c. of ground air, or 
nearly sufficient to give the increase of 1 in 10,000, and, as to the effects 
of diminished pressure, a fall of the barometer from 760 to 757 mm., or 
3 mm., would produce the same effect. 
Finally, let us suppose a fall in the barometer of 25 mm. (1 inch), 
and at the same time a rise in temperature of 10°C. ; the effect would 
then be an evolution of 3536 ¢.c. of ground air, which would be nearly 
equivalent to 18 of carbonic anhydride in 10,000 of air. 
It thus seems clear that changes of temperature and pressure, either 
alone or combined, should have a very distinct influence on the evolu- 
tion of ground air, which in its turn must distinctly affect the propor- 
tion of atmospheric carbonic anhydride, at all events in calm weather, 
and at heights not too remote from the surface of the ground. 
