in Hock Magmas on passing from liquid to solid. 243 



amples of sixteen series of electric measurements made with, 

 three kinds of rock magmas, which cooled slowly in a furnace 

 from the molten state. Temperatures* and resistances were 

 noted simultaneously every two minutes during each series. 

 The resistances were measured by Kohlrausch's method of 

 intermittent currents. The data for mean temperatures are 

 closely accordant, but at very high temperatures and at low 

 temperature sharpness is lost, owing to polarization and to the 

 immense range of electrolytic resistances encountered. These 

 difficulties are largely unavoidable, although our method is 

 capable of much improvement. Reductions to absolute values 

 are approximate. 



Table I. 

 Table 1. Tfie specific electrolytic resistances of rock magmas, varying with tempera- 

 ture and composition. 



1. Acid magma. 



2. Intermediate 



magma. 



?5-50 per cent Si0 2 . 



61-5 per cent SiOo. 



Tem- 





T em- 





Series. perature. 



Resistance. 



Series, perature. Resistance. 



xiv °C ohms XI 6 



xiii °C oh 



tisXIO 6 



1640 



1-8 



1430 



1-8 



1320 



2-2 



1260 



2-5 



1164 



2-9 



1114 



4-2 



1086 



3-1 



1034 



6-3 



1011 



3-8 



956 



8-8 



948 



4-7 



894 



13-4 



890 



6-5 



843 



20-0 



827 



8-0 



754 



46-4 



787 



11-6 



715 



70-2 



746 



15-3 



680 



105 



705 



21-6 



643 



158 



665 



29-5 



608 



239 



626 



41-3 



580 



344 



598 



53-8 



552 



497 



582 



64-2 



496 



1140 



560 



82-2 







533 



114 







504 



166 







467 



281 







444 



417 



- 





425 



584 







395 



1060 







355 



2540 







320 



5700 







- 3. 



Basic magma. 



48 - 5 per cent Si0 2 . 





Tem- 





Series. 



perature. 



Resistance. 



viii 



°C ohmsXlO 6 





1400 



2-9 





1230 



4-4 





1124 



5-4 





1050 



14-7 





995 



27'8 





939 



54'7 





883 



94-0 





834 



152 





792 



227 





754 



321 





723 



432 





692 



602 





645 



1000 





612 



1400 



The results of these observations have been plotted as in 

 the diagram, fig. 1, the ordinates being specific resistances in 



*Eegarding temperature measurements see Phil. Mag., July, p. 1, 1S92. 



