Geological History of the Atmosphere. 97 



carbonic acid annually evolved. If, then, there has been on 

 the average during recent geological epochs just enough car- 

 bonic acid of telluric origin to maintain the balance referred 

 to above, there must have been times when much more 

 was evolved than was required to make good the balance, 

 and there would also be times when there was not enough 

 evolved. Also, from considerations regarding the length of 

 time that volcanic activity prevails or continues in any par- 

 ticular district, we are entitled to infer that a period, during 

 which more telluric carbonic acid than is required for the 

 balance is added to the atmosphere, may last for a long 

 time — long enough to cause a very substantial increase in 

 the amount of carbonic acid present in the atmosphere. 

 Further, it is possible that periods during which there is a 

 deficiency may also last long enough to cause a serious di- 

 minution in the amount of atmospheric carbonic acid. It 

 is thus evident that in this way very considerable variations 

 may have taken place in the percentage of atmospheric 

 carbonic acid during recent geological epochs; and it is 

 quite possible that these variations may account for the 

 climatic changes of the occurrence of which there is an 

 abundant and striking body of evidence, and the causes of 

 which have caused much discussion among geologists and 

 astronomers. 



As far back as 1845 or so it was pointed out by Ebelmen 

 (as quoted by Dr. Sterry Hunt) that the greater weight of an 

 atmosphere charged with carbonic acid would increase the 

 temperature due to solar radiation at the earth's surface, and 

 greatly modify atmospheric phenomena. Later on, Tyndall 

 by his researches on radiation showed that certain gases have 

 a very considerable heat-absorbing power; so that a small 

 quantity of one or more of such gases present in the atmo- 

 sphere might have a very considerable effect on climatic 

 conditions. He even suggested that questions relating to 

 the climate of different geological epochs might find their 

 solution in the presence in the atmosphere of moderate 

 quantities of a gas like carbonic acid. 



More recently the subject has been investigated by 

 E. Lechner of Vienna and 8. Arrhenius of Stockholm. 

 Lechner announced as the result of his investigations (Phil. 

 Mag. eer. 5, vol. ii. 1881, p. 76) that carbonic acid rather 

 than aqueous vapour i> the constituent of the atmosphere 

 which absorbs the sun's radiation ; and he was able by his 

 method of working to calculate the percentage of carbonic 

 acid in the atmosphere, finding it 3'27 parts in 10. 000 parts 



Phil. May. S. 6. Vol. 9. No. 49. Jan. 1905. II 



