o54 Diurnal Variation of Carbon Dioxide in the Air. [Apr. 29, 



unsurrounded by vegetation the diurnal difference is as high as 0"64 

 vol. C0 2 in 10,000 of air. These resnlts compared with that obtained 

 at Grasmere — a diurnal variation, namely, of 0'34 vol. C0 2 in 10,000 

 of air — seem to point to some cause of variance nnconnected with 

 either climate or local conditions. 



And now that the existence of a dinrnal varip^tion may be said to 

 be fnlly established, the question as to npon what the difference 

 observed depends still remains to be answered, That in the main it 

 is due to plant action seems more than probable. And in order to test 

 the accepted theory that plants absorb carbonic acid during the day 

 and exhale it at night — to establish which must clearly be a first step 

 in the evidence in proof of a connexion between vegetable activity and 

 a diurnal variation in the quantity of atmospheric carbon dioxide — a 

 short series of experiments, ten in all, was made at Grasmere con- 

 currently with those an account of which has already been given. 



But before entering into a description of these it will be useful to 

 refer to the somewhat recent experiments of M. Reiset* already 

 mentioned, which were made in the neighbourhood of Dieppe in 1873, 

 inasmuch as they directly bear upon the point in question. He, 

 using Pettenkofer's method with aspiration, found that, in air taken in 

 the daytime over a field of red clover in the month of June, there were 

 only 2'898 vols. C0 2 in 10,000 of air present as compared with 2'915 

 vols, in air taken simultaneously in the open ; and that in the air over 

 a barley field in July, there were only 2"829 vols, as compared with 

 2'933 vols, in that of the open ; or a reduction in the air taken over 

 the crops, in one case of 0'017 vol. of carbonic acid, and in the other of 

 0"104 vol. in 10,000 of air as compared with that taken in the open. 

 Nevertheless, in a leafy coppice he obtained what at first sight 

 appeared to be somewhat contradictory results. In this position the 

 air gave a larger quantity of carbonic acid than that simultaneously 

 taken in the open. That taken in the coppice was found to contain 

 2*997 vols, in 10,000 of air, while that of the open only yielded 2'902 

 vols, in the same quantity of air. The explanation of this seeming 

 anomaly may possibly be found in the fact of a diminished quantity of 

 light existing within the coppice. 



For the purpose of the supplementary experiments to which refer- 

 ence has been made, a fine and healthy young geranium plant was 

 selected, due regard being had to the extent of its leaf-surface. This 

 was suspended day and night alternately in a jar of air sufficiently 

 large to contain it, the mouth being closed as already .described by 

 one of Jennings' capsules. 



The times chosen for the experiments were from 7 a.m. to 7 p.m. 

 for the day, and from 7 p.m. to 7 a.m. for the night. When the plant 



* " Compt. Eend.," 88, 1007-1011. 



