22 Scientific Proceedings, Royal Dublin Society. 



The " atmosphere," properly so called, would be that sum minus 

 (b+c). 



Now one of the most interesting deductions arrived at from 

 the examination of the fossil remains of the different geological 

 formations is, that our atmosphere has certainly varied both in 

 constitution and (most probably) in quantity, and if my assump- 

 tion of an intimate relation of the atmosphere with contraction be 

 correct, or admissible, there must have been periods or phases of 

 marked contraction, and therefore of very active vulcanism and 

 accompanying emission of gases, during or about these periods. 

 Such geological data as we already possess certainly do point to 

 periods of great volcanic activity, manifested by outbursts of lava 

 and alterations of the earth's surface, and corresponding changes 

 in the relations of land and ocean. The tertiary period may be 

 cited as an example in this respect. 



Turning now from what may, perhaps, be considered as mere 

 speculation, to the phenomena of the Krakatoa eruption, it 

 will be easily understood that, influenced by the considerations 

 already developed, my attention was particularly drawn to- 

 wards the role of the gases in this case, and that I was led to 

 attribute to them an importance proportional to the magnitude of 

 that event. Moreover, that very magnitude seemed to promise 

 some phenomena of a nature capable of demonstrating that actual 

 additions have thus been made to the atmosphere, and a due 

 consideration of the details furnished of the event lead, in my 

 opinion, very distinctly to that conclusion. 



From the very careful reports made by their engineers to the 

 Dutch Government, as well as from other sources, we have an 

 estimate given us of the quantity of ashes emitted, which had 

 fallen in such proximity to the locality, as to allow of an approxi- 

 mate measurement being attempted. Thus in the report which 

 appeared in Nature, vol. xxx. p. 10, the author says : " I found 

 that on calculating as accurately as possible the quantity of ejected 

 solid substances, they reached 18 cubic kilometres as a safe esti- 

 mate. These 18 kilometres represent a weight of more than 

 (36 x 10 12 ) kg." He adds: "the volume of ejected gases was, 

 perhaps, hundreds of times greater." Furthermore, he says : 

 "However large the quantity may be, it does not nearly reach 

 that which Tamburu produced in 1815, which Junghuhn estimated 



