1903.] The Cosmical Function of the Gh*een Plant. 429 



1-5 mm., so that it may be easily introduced into eudiometers of 2 mm. 

 inner diameter. This method has also the advantage that no stopcock 

 or rubber is used. 



All the necessary manipulations may be thus briefly summed up ; 

 thanks to this modified form of Bunsen's gasometer, the necessary gas 

 mixture on its way through this burette is, so to speak, cut into slices 

 of suitable volume and distributed to the leaves in the flat tubes. After 

 the necessary exposure to the spectrum the gases are extracted by 

 means of the pipette just described, and introduced for analysis into 

 these eudiometric tubes, which easily allow the estimation of 1/1000 of a 

 cubic centimetre. 



Thanks to all these contrivances, the problem of obtaining trust- 

 worthy analytical data with green surfaces exposed to a pure spectrum 

 was for the first time solved, and I still consider this method the only 

 one that may be safely relied upon in the study of this question. The 

 two other methods that have been proposed are far less exact. Count- 

 ing the number of gas bubbles emitted by small aquatic plants, which 

 may be demonstrated as a pretty lecture-room experiment, even if their 

 diameters are measured under the microscope, as recently proposed by 

 Kohl, is hardly to be recommended. It is true, we may obtain much 

 better results when using what I call my microeudiometer a small piece 

 of apparatus which permits us to measure and analyse in a couple of 

 minutes a bubble not bigger than a pin's head. Though reduced here 

 to a minimum, the sources of error inherent in the aquatic nature of 

 the plant are still not fully eliminated. 



As to the third method, Engelmann's well-known bacterium method, 

 I still consider that it is of rather too indirect a character, and 

 persist in my opinion, expressed many years ago, that chemical pro- 

 blems ought to be studied by chemical means. Moreover, we shall 

 presently see how little this method is to be relied upon. 



Thus we see that the introduction of a gasometrical method allowing 

 the measurement and analysis of very small quantities of gas made it 

 for the first time possible to study the assimilation of carbon in a pure 

 spectrum.* 



There were two more points to be considered before handling this 

 problem of the connection between the photo-chemical process and the 

 absorption of light in the green leaf. The spectra of chlorophyll and 



* In order to illustrate the difficulties I had to contend with, I may quote one 

 of the best authorities in this line of research, M. G. Lemoine. Speaking at so 

 recent a date as 1895 (' Annales de Chimie et de Physique,' Ser. 7, vol. 6, December) 

 of his experiments on the decomposition of oxalic acid in sunlight, M. Lemoine 

 says : " Les experiences devraient etre faites, non seulement avec la lumiere blanche, 

 mais encore avec ses differentes radiations. En pratique, il est impossible de les 

 isoler completement, et si on le pouvait, on riaurait plus assez d'intensite pour des 

 mesures quantitative.? ." My method may be thus summarised : a heliostat with the 

 largest mirror and a gas burette of the smallest bore. 



VOL. LXXII. 2 H 



