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excess of carbonic acid gas over the normal, grew better than a 
similar plant grown in “ pure air,” and De Saussure at once set 
himself the task of determining “ the dose of carbonic acid gas 
which, mixed with air, most favours vegetation.” He gives details 
of his experiments into which it is unnecessary to go; his results, 
however, are of great interest, for he found that when the air used 
in his bell-jar consisted of one-twelfth C0 2 (8*3%) and when the 
plants were exposed to the direct rays of the sun—tempered when 
the light was too intense—the plants were always healthier than 
when grown in ordinary air, while that in the shade the slightest 
dose of carbonic acid gas over the normal added to ordinary air was 
at once injurious.” Seventy and eighty years later, Godlewski and 
Kreussler carried out a series of experiments on the same subject 
and arrived at precisely the same results. 
De Saussure acknowledges that Priestley “ was the first to 
recognise that leaves were able to purify air vitiated by combustion 
and respiration,” but admits that “ he did not explain the cause of 
this phenomenon.” Senebier, on the other hand, is credited with 
the discovery of the fact that “ leaves decompose carbonic acid 
gas, appropriating its carbon and giving off its oxygen,” although it 
would seem that Ingenhousz has an equal if not a prior claim to the 
credit of this discovery. De Saussure deals with this—the funda¬ 
mental photosynthetic problem—in considerable detail, using as 
his materials plants of Vinca, Mentha, Lythrum, Pinus and Cactus. 
One of his conclusions is that “ plants in decomposing carbonic 
acid gas, assimilate a part of the oxygen contained in it.” He is 
more successful with his attempts to prove that “ plants fed on pure 
water and growing freely in air, obtain their carbon from the small 
amount of carbonic acid gas normally present in the atmosphere.” 
After a lengthy discussion of the gaseous exchanges exhibited 
by succulent plants like Opuntia and by different foliar types, by 
roots, woody stems, flowers and fruits, he summarises his results in 
the following words, “ Green plants growing freely in air and 
exposed to the successive influence of light and darkness, alternately 
inspire and expire oxygen mixed with carbonic acid gas. Green 
plants do not immediately assimilate the oxygen they take in ; on 
inspiration it is changed into carbonic acid gas. This is decomposed 
on expiration and it is only through this (partial) decomposition 
that they canjassimilate the oxygen which is in the air. Speaking 
generally, non-green parts do not exhibit these successive inspira¬ 
tions and expirations, they neither directly nor indirectly assimilate 
