AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 255 



That the .sodium chloride could not exist as such is shown in the 

 eveiyday operations of the pottery kiln, whereby common salt is 

 decomposed in presence of water vapor, the sodium uniting- with the 

 ox3"gen of the water vapor and the heated earthenware to form sodium 

 aluminum silicate, and the chlorine with the hydrogen of the water 

 vapor 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, andhj^dro. 

 chloric acid. This pressure can not, however, be supposed to have 

 influenced the chemical combinations occurring in the liquid silicated 

 magma of the earth's surface. 



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

 say 1,000° C. was attained, we observe that water vapor 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 con- 

 siderable intermingling of layers probably previously differentiated 

 by specific gravity acting on a mass which was hardly likely to be mole- 

 cularl}'^ 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 behavior of silicates at high temperature show 

 that these bodies are stable for the most part, certainly up to 1,500° 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 crystalline differentiation had progressed, does not much con- 

 cern 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 



^"On the secular eoolinfi of the earth," and "On the rigidity of the earth." 

 Mathematical and Physical Papers, Vol. III. See also Green's Physical Geology, 

 1882, p. 655. 



