FERGUSON: OXIDATION OF LAVA 539 



response prevailed. On 33 volts the galvanometer deflection 

 was entirely negative and eight times as large as the observed 

 maximum purely positive deflection for 10 volts. In another 

 test, at a slightly lower temperature ( — i04°C.), which raises 

 the critical voltage, changing from 35.6 volts to 37 volts trans- 

 formed the positive-negative deflection of =*= ^ tnm. into a nega- 

 tive deflection of — 230 mm. 



These tests were carried out at — ioo°C. to — i78°C. It 

 is of interest to note that the photo-positive action is the same as 

 a resistance decrease caused by a rise in temperature of the 

 material, while the photo-negative action is similar to the build- 

 ing up of a counter-electromotive force (the electrolytic action), 

 previously observed in silver sulfide. An equal-energy spectrum 

 was used for the radiation stimulus and if this phenomenon 

 were the result of heating and of electrolytic action, then the 

 photo-positive response should occur in the short wave-lengths 

 where the absorption is greatest, and the photo-negative re- 

 sponse should occur in the long wave-lengths where the photo- 

 electric activity is the greatest. This is just the reverse of what 

 has been observed. 



No explanation of this phenomenon is attempted at this 

 time. Even if it is "only a gas effect," "electrolytic action" 

 or "surface charge," it is unique in being selective as to the wave- 

 length of the exciting radiation and in being photo-positive or 

 photo-negative, depending upon the applied voltage. It there- 

 fore requires further investigation. 



GEOCHEMISTRY. — Tlte oxidation of lava by steam. J. 

 B. Ferguson, Geophysical Laboratory, Carnegie Insti- 

 tution of Washington. 



Crystals of olivine and other iron-bearing minerals are found 

 associated with glass in the normal Kilauean lava, and some 

 years ago the question arose as to whether their presence might 

 be taken as evidence bearing upon the probable water content 

 of the volcanic exhalations. Preliminary experiments^ made 



1 Day, a. L., and Shepherd, E. S., Water and volcanic activity. Bull. Geol. vSoc. 

 Amer. 24: 602. 1913. Smithsonian Report for 1913, 302 (Publication 2286). 



