20 Scientific Proceedings, Royal Dublin Society. 



suns, gases play a very important part, if they be not the only con- 

 stituents. For certain of these bodies it has been shown that car- 

 bon, and hydrogen, and hydrocarbons, are essential elements, both 

 chemically and physically. Are we not, therefore, entitled to as- 

 sume that these elements and combinations were abundantly pre- 

 sent, and very active agents, in the first stages of the development 

 of the earth, and if so, that traces of their influence may still be 

 found both in our atmosphere and in the interior of the globe ? 

 Is it not reasonable to suppose that, in the slow and continuous 

 process of contraction, very great masses of gases became retained 

 or occulted by the cooling matter, and that these occulted gases 

 have been the essential agents in balancing tensions in the con- 

 tinually contracting sphere ? — that this continuous contraction led 

 to the pressure of masses of these gases until heat was liberated in 

 more or less degree, and frequently to the point which brought 

 into play chemical affinities ; and that thus the whole series of 

 phenomena, which have tended and are tending to modify the 

 form of the earth's surface, are intimately bound up with the 

 existence and action of gases in the interior and at the surface 

 thereof ? 



Thus, from the very earliest period, we are called upon to re- 

 cognize the continuous presence of gases as essential constituents 

 of the earth's mass, and, so far as analogy allows us to judge, as 

 most active agents and products of alteration. For the period dur- 

 ing which the crust was not yet formed they must have been pre- 

 dominating agents ; while for the subsequent periods, during which 

 the temperature decreased and the crust increased in thickness, their 

 intensity of action must have gradually diminished, and their emis- 

 sions become more and more spasmodic, or of longer period, until 

 conditions were established which we now designate as volcanic, 

 that is, when contraction could only take place by reason of the 

 sinking of masses of the crust, with accompanying vulcanism and 

 earthquake phenomena, such as we witness at the present time, one 

 of the most important and constant of which is the emission of 

 gases. 



Leaving aside speculation as to the initial constitution, volume, 

 and state of the atmosphere, and coming down to the period during 

 which the earliest stratified rocks were being formed, we are led to 

 imagine for that period a globe greater in diameter than at present, 



