REPORT OF THE CHIEF ASTRONOMER /37 



SESSIONAL PAPER No. 25a 



k. 



Silver, about 1-0000 



Copper, about -9480 



Lead -0836 



Quartz -0158 



Marble -00817 



Granite -00757 --0O975 



Gneiss -000578 - -00817 



Sandstone -00304 - -00814 



Basalt -00673 



Syenite -00442 



Glass -00108 - -00227 



Water, about -00130 



Paper , -00031 



Flannel .' -00023 



Silk.. ..' -00022 



Cork -00013 



Feathers -0000574 



Weber has found that, h for gneiss at 0° C. is 0-000578 and at 100° C. 

 0-000416, showing a very great lowering with increase of temperature.* In fact, 

 through the interval 0° - 100° C, h seems to vary about inversely as the absolute 

 temperature.f It is not impossible that the conductivity of rock at 1,100° C. 

 approaches that of water, famous as a poor conductor. Thorough experimenta- 

 tion on this subject is urgently needed. 



In .the present connection the thermal diffusivity (k) of rock, rather than 

 its conductivity, is of first importance. If s = specific heat and d = density, we 

 have 



K ~^d 



For rock at room temperature (20° C.) Kelvin assumed 400 as the value of k when 

 the unit of length is a foot, the unit of time a year, and the unit of temperature 

 one degree Fahrenheit. This value is close to that which represents the average 

 of the determinations made for different rocks at room temperatures, during the 

 years since Kelvin wrote his famous essay4 



If k be assumed as 400 at all temperatures up to 1300° C, it is possible 

 to calculate the temperature gradient in the wall-rock of a molten batholith at 

 the end of specified periods of time. For practical purposes the surface of contact 

 may be regarded as infinite; let it further be considered as plane. Under these 

 conditions the following Fourier equation furnishes the datum for calculating 

 the temperature at a point x feet from the contact at the end of t years, if the 

 magma is kept stirred by currents. § In the equation b = the temperature of the 



* Forbes and Hall have proved analogous relations for iron and for magnesium 

 oxide; cf. J. D. Forbes, Trans. Roy. Soc. Edinburgh, Vol. 24, 1867, p. 105, and E. H. 

 Hall and others, Proc. Amer. Acad. Arts and Sciences, Vol. 42, 1907, p. 597. 



t Cf. P. G. Tait, Recent Advances in Physical Science, 2nd ed., London, 1876, p. 270. 



% Trans. Roy, Soc. Edinburgh, 1862. 



§ Cf. W. E. Byerly's Elementary Treatise on Fourier's Series, Boston, 1893, p. 86. 



