Joty—An Estimate of the Geological Age of the Harth. 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 chlorine 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 800 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 Karth’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 gravity 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 comparatively 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 Harth at the period 
we refer to, it is probable that much volatile matter was held in solution in the 
melted magma, and ultimately trapped im the solid crust. How far this was 
a glass, or how far crystalline 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 Karth’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 
* ¢¢Qn the Secular Cooling of the Earth,” and ‘‘On the Rigidity of the Earth.” Mathematical and 
Physical Papers. Vol. ur. See also Green’s ‘‘ Physical Geology,” 1882, p. 655. 
TRANS. ROY. DUB. SOC., N.S. VOL. VII., PART ILL, G 
