JoLY — An Estimate of the Geological Age of the Earth. 31 



operations of the pottery kiln, whereby common salt is decomposed in presence of 

 water vapour, the sodium uniting with the oxygen of the water vapour, and the 

 heated earthenware to form sodium aluminium silicate, and the chloi'ine with the 

 hydrogen of the water vapour to form hydrochloric acid. The glaze produced in 

 this way on the earthenware is highly insoluble. 



Under this condition of temperature a gaseous pressure, of not less than 300 

 atmospheres — probably between 300 and 400 — must have obtained, due to the 

 oxygen, hydrogen, carbonic anhydride, and hydrochloric acid. This pressure 

 cannot, however, be supposed to have influenced the chemical combinations occur- 

 ring in the liquid silicated magma of the Earth's surface. 



If we transfer our attention to a later epoch, when a temperature of, say, 

 1000^ C. was attained, we observe that water vapour would be stable, and a crust 

 would be forming upon the surface of the Earth. We find now events progressing 

 in this early solid crust which have already been indicated by Lord Kelvin.* 

 The break up and submergence of the denser solid constituting the crust would 

 certainly lead to a considerable intermingling of layers probably previously 

 differentiated by specific gi-avity acting on a mass which was hardly likely 

 to be molecularly homogeneous throughout. We must note, however, that 

 this action can only have extended to comjoaratively shallow depths, as such 

 descending fragments would soon find themselves buoyed up and re-fused by 

 the denser magma beneath. 



Observations on the behaviour of silicates at high temperature show that these 

 bodies are stable for the most part, certainly up to 1500° C, but upon complete 

 fusion readily yield up included or combined water. Still, under the conditions of 

 pressure and temperature obtaining at the surface of the Earth at the period 

 we refer to, it is probable that much volatile matter was held in solution in the 

 melted magma, and ultimately trapped in the solid crust. How far this was 

 a glass, or how far ciystalline differentiation had progressed, does not much 

 concern the present issues, and is, in any case, difficult or impossible to determine. 



We now transfer our attention to yet another period of the Earth's' early 

 history. An eventful period, when the temperature near or at the surface had 

 fallen to the critical temperature of water, 370° C. At this temperature a pressure 

 of 196 atmospheres would suffice to liquefy it. The pressure was very probably 

 much above this even at points high up in the atmosphere. 



When this critical temperature was attained at such a point in the atmosphere 

 as to be attended by pressure conditions exceeding the critical pressure an instant 

 change of state occurred. The water resulting — almost still a vapour, but pos- 

 sessing a surface, although a highly energetic one — probably floated in the equally 



* "On the Secular Cooling of the Earth," and " On the Kigidity of the Earth." Mathematical and 

 Physical Papers. Vol. iii. See also Green's "Physical Geology," 1882, p. 655. 



TfiANS. EOT. DUB. SOC., N.S. TOL. VII., PAIil Ul. G 



