Lerrs & Bhake—The Carbonic Anhydride of the Atmosphere. 178 
Secrion I].—Causzs oF VARIATION. 
In attempting to review and criticise the subject of the variations 
in the amount of atmospheric carbonic anhydride many difficulties 
are encountered, and perhaps not the least of these is to correctly 
appraise and estimate the value of the evidence brought forward 
in support of the different contentions which have been raised from 
time to time in this matter. 
We have seen that it took at least seventy years for chemists to 
arrive at a correct estimate of the average amount, and that during 
that time the figure has dwindled down from 100 to 200 vols. in 
10,000, to about 3. 
As most of the later authorities agree that the variations in this 
amount which are caused by natural agencies are small—the 
resources of the most reliable of our present methods of determi- 
nation are taxed to their utmost, and therefore it is not surprising 
to find a conflict of opinion among those who are entitled to speak 
with authority on the subject, not only as regards the causes of the 
variations but as to their extent also. 
Two opposite views have been maintained, of which De Saussure, 
and Gay Lussac were the respective authors. According to the 
first of these, considerable variationsin the amount of atmospheric 
carbonic anhydride occur, of a more or less lasting or permanent 
nature, caused by natural agencies such as the seasons, influence of 
vegetation, the direction and force of the wind, humectation and 
desiccation of the soil, day and night, &e. 
Gay Lussac, on the other hand, was of opinion that owing to the 
continuous movements of the air, both horizontally and vertically, 
practically uniform distribution or diffusion of the carbonic 
anhydride occurs. It must be at once granted, from the evidence 
which has since been forthcoming that Gay Lussac’s opinion is cor- 
rect to the extent that the variations are much less than De Saussure 
and others of his school believed and that the average amount of 
atmospheric carbonic anhydride is much the same under the most 
diverse conditions of weather, locality, season, &c. 
But on the other hand, there can be no doubt that variations in 
the amount do occur; that, toa certain extent, they are lasting and 
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