Letts & Buaxe—The Carbonic Anhydride of the Atmosphere. 209 
in the atmosphere and in ground air follow each other very closely 
during the period January to May. From May to July the in- 
erease in the carbonic anhydride of the ground air continues while 
thet of the atmosphere diminishes, and the most probable explanation 
is the great activity, during this period, of vegetation in decom- 
posng the gas, and thus overcoming the increased amount stream- 
ing out of the ground. From October to January an increase 
ocewrs in the atmospheric carbonic anhydride and a decrease in 
thatof the ground air. The latter is, no doubt, due to the gradual 
lowering of temperature and decrease in the activity of the micro- 
orgwisms of decay. The former is more difficult to account for, 
but the cessation of vegetable activity may possibly be the cause. 
Influence of Cloud and Sun. 
Intimately connected with the effects both of day and night, 
and of vegetation, are those which should be produced by cloud 
and sun respectively ; and here, too, the factor of the seasons 
should come into play. 
During bright, sunny weather in summer it might be antici- 
pated that owing to the resulting activity induced in vegetation 
in decomposing carbonic anhydride, a diminution of that gas 
would occur in the atmosphere over a land surface. ‘That this 
effect actually is produced has been maintained by some of the 
chief observers. Thus Reiset says—‘‘ The minima correspond 
with cloudless days and bright sun”; and Mintz and Aubin— 
“‘The proportion of carbonic anhydride is at its minimum when 
the sky is clear and the air agitated. It is at its maximum during 
cloudy and calm times. It is easy to put in evidence the varia- 
tions which are produced by these influences on the same day. 
Hxample :— 
April Ist, 1881, 9 a.m. sky clear, airagitated, 2°73 
if alk Oh ena ee er covereds |. 200 
% » 4.0 p.m. ,, much covered, 
commencement of rain, . 2°99 
Russell, speaking of London air, says — “On examining 
Table 7 it will also be seen that it very often happens that the 
amount of carbonic acid is very considerably below the average, 
