1911.] The Mechanism of Carbon Assimilation. 107 



small quantity (about - 2 c.c.) of oxygen was found to be present, and was 

 detected by absorption in alkaline pyrogallol, a result which appears to 

 confirm the statement that the initial stage of photosynthesis can be 

 carried on to a small extent quite independently of living protoplasm. 

 This evolution of oxygen by dead tissues was indeed observed by Molisch* 

 in 1904, in the case of foliage leaves of Lammm album, by the luminous 

 bacteria method. 



3. The Absorption of Heat in a Chlorophyll Film, due to Photolysis of 

 Carbon Dioxide. 



It has now been shown that carbon dioxide in the presence of water can 

 be decomposed by ultra-violet light without chlorophyll, and that the same 

 decomposition products can be obtained by the action of ordinary light 

 when chlorophyll is present. Thus there is at least a strong probability 

 that carbon dioxide, and not any constituent of the chlorophyll, is the 

 parent of these decomposition-products. The remaining link in the 

 argument has, however, been supplied by the application of a thermometric 

 test, which is described below. It is clear that, if the assumption is correct, 

 a chlorophyll film in an atmosphere containing moist carbon dioxide should, 

 when illuminated, remain at a lower temperature than a similar film equally 

 illuminated in an atmosphere devoid of that substance, for the production 

 of formaldehyde and hydrogen peroxide from aqueous carbon dioxide involves 

 the absorption of a large amount of heat. 



The arrangement devised was a differential one, and served to show the 

 difference between the temperatures of two chlorophyll films set up side by 

 side in two glass tubes which contained the desired gaseous mixture. The 

 apparatus used in the final series of measurements is diagrammatically 

 represented in fig. 2. The chlorophyll films were painted on pieces of thin 

 tinfoil about 1 cm. square {a, a'), which were gummed on to strips of cork 

 (b, b'\ which fitted closely in the glass tubes. A single thermo-electric 

 junction was hammered to the back of each piece of tinfoil before the 

 latter was fixed to the cork, and the leads from the junctions, double silk 

 covered and soaked in shellac varnish, passed out at the top of the tubes 

 through a narrow thickened portion of the glass, the passage being sealed 

 by pouring melted paraffin wax into the cups at c, c'. The thermocouples 

 were of copper constantan wire (S.W.G-. No. 36), and the weight of the metal 

 substratum of the film, including the hammered- on thermocouple, was about 

 - 02 grm. per square centimetre. The cork strips served to support the 



* 'Bot. Zeit.,' 1904, vol. 62,ip. 1. 



