RELATIONS TO OTHER SCIENCES 83 



quite possible that if a living organism were cooled only to temper- 

 atures at which physical changes, such as crystallization, take place 

 with reasonable velocity, the process would be fatal, whereas, if they 

 were cooled to the temperature of liquid air no such change would 

 take place within finite time, and the organism would survive." 



Also the study of the various combinations of carbon and iron 

 that may exist in steel, and the conditions of equilibrium that 

 exist between them has proved a most important investigation in 

 the field of what van 't Hoff calls solid solutions. 



Geology, dealing as it does with the greatest variety of physical 

 processes, such as changes of state, fusion, crystallization, solution, 

 conduction of heat, radiation, with complications depending on 

 variations of pressure and temperature, presents many problems 

 for the solution of which the resources of modern physics must be 

 taxed. The fusing-points of the different chief minerals of the earth's 

 crust, the effect of great pressure on their fusing-points and modes 

 of crystallization, the crystallization of the various elementary min- 

 erals out of a fused magma also studied at different pressures, the 

 effect of pressure not only on fusing-points, but .on the viscosity and 

 rigidity of minerals at high temperature, the heat conductivities of 

 the various substances making the bulk of the earth's crust, all these 

 are questions that must be thoroughly studied to enable the geologist 

 to determine the probable condition both of temperature and pres- 

 sure which prevailed during the formation of a given rock mass, and 

 to throw light on the great problem of geology, the age of the earth. 



To this latter question, physics has already given a tentative 

 answer. Lord Kelvin's discussion, based on the assumption of the 

 earth as a mass cooling from a uniform high temperature, points 

 to a period of between twenty and one hundred million years, within 

 which geologic changes in the crust of the earth must have occurred; 

 while Helmholtz and Kelvin's deduction of the time during which 

 solar radiation can have been of such an intensity that life conditions 

 on the earth were possible gives about twenty million years as the 

 limit. 



But later investigations giving new data as to the properties of 

 the materials of the earth's crust, as to the laws of variation of radi- 

 ation with temperature, and as to absorption and radiation by the 

 solar and earth's atmospheres, will all contribute to modify and make 

 more precise these methods. Already some progress in this direction 

 has been made. A few years ago, Clarence King gave a most inter- 

 esting and ingenious rediscussion of Kelvin's cooling of the earth 

 method, making use of the determinations made by Barus of the 

 fusing-points of diabase at different pressures, and gives as the most 

 probable result of the method the period of twenty-four million 



