RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 25 



property in the deposits from the thermal springs at the Institute of 

 Bogni di Lucca, in Tuscany. 



Tomassina ^^^ detected radioactivity in the lava thrown out by 

 Vesuvius in the eruption of 1904, and, in a study of 28 samples of 

 igneous rocks, including granite, basalt, hornblende, and serpentine, 

 Strutt ^^"^ found radium present in all. More was found in granite 

 than in any other igneous rock studied, while the basic rocks con- 

 tained the least amount. Iron meteorites were found to contain very 

 little, if any, while stone meteorites contained about as much as the 

 terrestrial rocks which they resembled. 



The same author ^^"^ found radium present in sedimentary rocks 

 generally, marble, chalk, flint, clay, roofing slate, oil-bearing sand- 

 stone, deposits from the hot springs of Bath, sea salt, boiler-rust 

 (Cambridge, Eng.), and in the rock-forming minerals, zircon, apatite, 

 hornblende, tourmaline, labradorite, white feldspar, white and brown 

 mica, white quartz and others. The specimens came from various 

 widely separated regions in continental Europe, Africa, India, Asia 

 (Ural Mts.), England, and the United States. It was found that 

 more than one half of the radium is contained in the heavy minerals, 

 though these form only about one eighth of the whole mass of the 

 rock. 



The association of radioactivity with the *' ashes" and lava of 

 Vesuvius was reported by Becker,^" and a slight radioactivity of 

 soils, clays, basaltic tufas, basalts, soft calcium carbonate, etc., 

 was detected by Accolla^ in 1907. Sands and mud of the seashore, 

 and mud from the sulfur spring of Brucoli also possess a weak 

 activity. 



Analyses by Eve and Mcintosh ^ showed in sedimentary Ordo- 

 vician rock from .92 x io~^^ to .91 gr. of radium per gram of 

 rock; in igneous Devonian .26 to 4.3 gr. per gram of rock, and in 

 sedimentary Quaternary .16 to .8 gr. per gram of rock; and the 

 probability that the internal heat of the earth is due to radium is 

 discussed /ro and contra by H. A. Wilson, ^^^ Strutt,^^^ and others. 



Radioactivity in Air: Elster and Geitel ^^' *'' were the first to 

 show that the atmosphere contains a radioactive emanation. They 

 suspended a negatively charged wire for some hours in the air, then 

 coiled it up and tested it with the electroscope. The wire was found 

 to be radioactive, but this result was not obtained if the wire was first 

 given a positive charge. Now a negatively charged wire immersed 



