in Sock Magmas on passing from liquid to solid. 247 



can be exposed, the extreme sensitiveness of the electrical 

 indications, and the simplicity of the necessary apparatus, 

 suggest a method of pyrometry specially available for tempe- 

 rature approaching the melting point of platinum. One 

 would obviously select an exceedingly acid magma, both be- 

 cause of its relatively great infusibility, and because of the 

 occurrence of the maximum of uniform electrical conduction, 

 in such a case, as will be shown in the next paragraphs. 



§ 5. Relations to chemical composition. — Endeavoring to 

 interpret these results with reference to their chemical bear- 

 ing by means of Kohlrausch's* law we are handicapped at the 

 outset because the rates of procession of ions (Hittorf's 

 " Uberfuhrungszahlen ") in silicate solutions are not known. 

 In other words the quantities expressing the velocity of either 

 ion relative to the combined velocities of both are unknown. 

 We cannot even state what the structure of the electrochem- 

 ical equivalents may be. It makes a difference whether the 

 cations travel singly or in groups, or whether the anions be 

 ijSi0 3 , ^Si0 4 , etc., singly or combined. We could not there- 

 fore express more than a mere opinion relative to the velocities 

 of the ions, even if the specific molecular conductivities were 

 computed. Nevertheless, in the face of these difficulties we 

 are able to deduce certain striking results insomuch as the 

 method of this paper is one in which the cation is brought 

 gradually, and as nearly as possible to vanish in amount. An 

 index of acidity, independent of temperature, is given by the 

 constant h of equation (1). We regard the anions, whatever 

 their specific structure may be, as identical in the three mag- 

 mas examined ; and the cations, from the large number of 

 elements contained in the magmas, as possessed of a common 

 or average property. 



Several general deductions, however, may be drawn from 

 the work, incomplete as it is at present. In the first place, 

 Table I shows strikingly that electric conduction increases with 

 the degree of the acidity of the magma, that is, with the 

 degree of dilution of the cation. And since fusibility de- 

 creases in a marked way as the composition of the magma 

 approaches pure silica, it follows that in a series of different 

 magmas electric conduction at any given temperature increases 

 in proportion as the viscosity increases. 



Thus the most acid of the rocks investigated, (1), is a stiff 

 paste above 1500,° whereas the basalt, (3), can be poured at a 

 temperature much below 1300°*; and yet the rhyolite, in vir- 

 tue of its acidity is a better conductor than basalt at any given 

 temperature. 



*Kohlrausch, Wied. Ann., vi, pp. 145 et seq., 1879. 



