Geological History of the Atmosphere. 315 



The results obtained were very striking and important, both 

 as regards the way in which plants were found to thrive 

 under very unusual conditions, and also the very great 

 difference that was found to exist between different kinds of 

 vegetation as regards their rate of producing oxygen; that is 

 to say, the amount of oxygen produced by the same or 

 similar weights of different kinds of plants in the same time and 

 under similar conditions. As regards the experiments with 

 gases, a most remarkable result was obtained in the case of 

 hydrogen gas. The plant grew perfectly well, being of course 

 at the same time supplied with carbonic acid in aqueous 

 solution and with the elements of a fertile soil ; but in the 

 course of the experiment the hydrogen entirely disappeared. 

 Dr. Phipson considered that it must have combined with the 

 nascent oxygen produced by the decomposition of carbonic 

 acid by the vegetation under the influence of light. He also 

 infers from the same experiment that there was no free 

 hydrogen in the earth's primitive atmosphere ; but this in- 

 ference is surely much more than is warranted by the facts 

 observed. It would no doubt be fair enough to infer that if 

 there was vegetation growing in very ancient times in an 

 atmosphere containing hydrogen gas, the oxygen liberated 

 from carbonic acid by the vegetation would not remain 

 permanently free so long as there was any free hydrogen left. 

 But vegetation may quite well have continued to grow in an 

 atmosphere containing hydrogen until all the hydrogen had 

 disappeared through combining with the oxygen produced 

 by the vegetation, and after that time the oxygen liberated 

 by vegetable growth would begin to have a chance of re- 

 maining permanently free. 



The general interest taken in the question of the history of 

 free oxygen has increased considerably within the last two or 

 three years on account of the opinions expressed by Lord 

 Kelvin in 1897. In that year he discussed the subject on 

 two separate occasions, viz., in his address to the British 

 Association in Toronto on the Fuel Supply and the Air 

 Supply of the World, and in his address to the Victoria 

 Institute, London, on the Age of the Earth*. In the 

 former of these addresses (as briefly reported in ' Nature,' 

 vol. lvi. p. 461) he argued that as the earth was in all proba- 

 bility originally hot and liquid, no primeval vegetable-fuel 

 existed ; further, no free oxygen existed at that period, 

 since it is not found in gases evolved from minerals or in the 

 spectra of stars. He considered, therefore, that the oxygen 

 of the air has probably resulted from the action of sunlight 

 * Phil. Mag. January 1899. 



