Page:Radio-activity.djvu/542

 foot of the American Fall at Niagara. An insulated wire was suspended near the foot of the Fall, and the amount of excited activity on the wire compared with the amount to be obtained on the same wire for the same exposure in Toronto. The amount of activity obtained from the air at Toronto was generally five or six times that obtained from the air at the Falls. In these experiments it was not necessary to use an electric machine to charge the wire negatively, for the falling spray kept the insulated wire permanently charged to a potential of about -7500 volts. These results indicate that the falling spray had a negative charge and electrified the wire. The small amount of the excited radio-activity at the Falls was probably due to the fact that the negatively charged drops abstracted the positively charged radio-active carriers from the atmosphere, and in falling carried them to the river below. On collecting the spray and evaporating it, no active residue was obtained. Such a result is, however, to be expected on account of the minute proportion of the spray tested compared with that present in the air.

279. A very penetrating radiation from the earth's surface. McLennan, and Rutherford and Cooke independently, observed the presence of a very penetrating radiation inside buildings. McLennan measured the natural conductivity of the air in a large closed metal cylinder by means of a sensitive electrometer. The cylinder was then placed inside another and the space between filled with water. For a thickness of water between the cylinders of 25 cms. the conductivity of the air in the inner cylinder fell to about 63 per cent. of its initial value. This result shows that part of the ionization in the inner cylinder was due to a penetrating radiation from an external source, which radiation was partially or wholly absorbed in water.

Rutherford and Cooke observed that the rate of discharge of a sealed brass electroscope was diminished by placing a lead screen around the electroscope. A detailed investigation of the decrease of the rate of discharge in the electroscope, when surrounded by metal screens, was made later by Cooke. A thickness of 5 cms. of lead