360 Mr Fisher, On the transmission of 



which implies that the volume of gas dissolved in a volume of 

 magma at the pressure just under the crust is 0*01254 of that 

 volume. 



If RV be the volume to which the gas in the volume V of 

 magma would expand on reaching the surface, where it will be 

 exposed to the atmospheric pressure only, then, by the expression 

 already found for the free gas, 



RV = rV(-- 



\p 



where P is the weight of the crust and p that of the atmosphere, 

 which is about that of 34 feet of water. 



This will give about 130 cubic feet of gas at atmospheric 

 pressure given off by every cubic foot of lava when it reaches the 

 surface of the earth. 



It must not be supposed however that this represents the 

 whole expansive force in a volcano, because throughout the entire 

 duct gas is being liberated, so that the expansive force is the 

 aggregate of the pressure of the liberated gas throughout the 

 height of the duct until the supply is temporarily exhausted. 



Our data enable us to find the value of e, the volume elasticity 

 of the magma : for, comparing the velocities of the two types of 

 waves, we have 



( ILY _ — 

 \f29J " P ' 



Now expressed in C.G.S. units 



P = 2-68 x 25 x 160933 x 98117, 



which with the value of r already found gives 



e = 3-647 x 10 12 . 



This is of the same order of magnitude as some values 

 I obtained for the compressibility of specimens of granite, calcu- 

 lating from values of Young's modulus, and the coefficient of 

 rigidity as measured by Dr H. Nagaoka and by Dr Knott 1 . 



We have been considering the magma just beneath the crust. 

 At greater depths P will be increased, and D will be increased, 

 and probably the temperature also, e will also probably be 

 increased. But if the interior is liquid, the temperature may be 

 more equable than if it was rigid. So long however as the 

 properties of the gas and liquid remain the same, r would be 

 constant. According to Laplace's law, the pressure increases 

 more rapidly than the density. The velocity will therefore 



1 "Eival theories of Cosmogony," by tbe Author, American Journ, Science, 

 June, 1901, 



