212 Notices respecting New Books. 



same time these results appear to be the legitimate deductions 

 from generally received geological data, now for the first time 

 submitted to mathematical treatment. The most important ad- 

 ditions will be found in Chapters v., yi., yiti., ix., xv., xyii., xviii., 

 XX., XXIII., xxv." 



I. Underground temperature is the subject of the opening 

 chapter. The general law of its average rise on descent into the 

 earth, and the probability that it does not remain unaltered for all 

 depths, are discussed. 



II. Internal densities and pressures. Chief points : — the Earth 

 probably once wholly melted ; now consisting of concentric shells ; 

 mean and surface densities ; internal pressure ; numerical value of 

 the densities according to Laplace's and Darwin's laws, — both 

 satisfying the geodetic and astronomical tests, but the former 

 being the most probable expression of the facts ; for, "if the den- 

 sities of the substances in the central parts of the earth are due 

 rather to their intrinsic nature than to condensation by pressure, 

 it is clear that Laplace's law is the more probable representation 

 of the reality; for it must be remembered that, although based 

 upon a supposed relation between density and pressure, it does not 

 necessarily imply that the density is the result of the pressure." 



III. The condition of the Interior is next considered. "The 

 question as to whether the Interior of the Earth is at the present 

 time solid or fluid, or partly solid and partly fluid, apart from geo- 

 logical considerations, may be attacked in two ways. The first of 

 these is by inquiring what would be the difference of effect that 

 bodies exterior to it, namely the sun and moon, would have upon 

 the motions of the Earth in either case ; and, secondly, by con- 

 sidering the sequence of events according to which a molten globe, 

 such as the Earth once was, may have passed into its present state." 

 In the first case arguments have been based on the phenomena of 

 the Precession of the Equinoxes and the Tides. These are briefly 

 discussed and disposed of in Chapter III., no appreciable tide in 

 the body of the Earth being produced by the attraction of the 

 moon and sun. The change of density on solidification is illus- 

 trated by many collected calculations and observations; and the 

 existence of a cooled crust over a molten mass is regarded as 

 possible. 



It is then argued that a liquid substratum might dissolve water- 

 gas, according to Henry's law of the absorption of gases by liquids, 

 and that there would be a very considerable quantity of steam 

 emitted from this substratum if it gained access to the atmosphere 

 in volcanos. Moreover, the high temperature of the gases (chiefly 

 water-gas) passing up through the lava would support its tem- 

 perature, and would also account for amygdules in the deep-seated 

 igneous rocks of many dykes. This hypothesis of absorbed gases 

 is regarded as accounting for the alleged absence of bodily tides 

 within the Earth. 



Chapter VI., with its text, — " a thin crust implies an energetic 

 substratum," — goes to prove that this substratum must be affected 

 by convection-currents, upward and downward, according to local 



