Three shallow cells 17 mm. in diameter and 2 mm. deep were filled with polonium subnitrate, the same quantity in each cell, the surfaces being levelled. Over these cells were fixed three pieces of lead pipe of lengths of 1, 2, and 4 inches, so that each cell of polonium had its emanations confined to its own tube. Sensitive films were put over- each tube, and the whole kept in total darkness for 144 hours. At the end of that stretch of time the films were developed.' No image was seen on the film 4 inches from the polonium, a faint image was perceptible on the film 2 inches off, and a stronger one on the film 1 inch off. Measured in the opacity meter the figures were :
Over 1 inch tube Opacity log. = (H8; Opacity = 1-51. Over 2 inch tube Opacity log. = O04; Opacity = (HI.
A repetition of this experiment, using tubes of glass instead of lead, gave almost identical results.
The electron theory explains a fact which has long puzzled experi- mentalists. It is well known that if a coin is laid on a sensitive plate in perfect darkness and connected with one pole of an induction coil for a few seconds and then developed, an image can be obtained of the raised parts of the coin. This has generally been explained by saying that the electrified stream of air, or the "brush discharge," affects the film like light.
But Mr. F. Sanford* shows that coins embedded in the centre of a block of paraffin 2 cm. thick, where they could not send off streams of electrified air, can still be photographed by means of the induction coil. Under these circumstances it is probable that electrons are the agents, as electrons will easily pass through paraffin wax from the coin to the sensitive plate, when the coin is connected with the negative pole of an induction coil, the other pole being connected with a metal plate placed below the wax block.
Hitherto we have been dealing with negative electrons a free positive electron at present is unknown. In a paper communicated to the Royal Society, December, 1900,t the Hon. K. J. Strutt offers a suggestion as to positive ions which in a satisfactory manner appears to explain much that hitherto has been left doubtful, not to say con- tradictory.
He adopts the generally recognised theory that the deflectable Becquerel rays consist of a stream of negative corpuscles Avith enor- mous velocities proceeding from the radio-active body. But there are two kinds of Becquerel rays, one deflectable and penetrating, the other non-deflectable and easily absorbable. Mr. Strutt considers that these non-deflectable rays are positive ions moving in a stream from the radio-active body.
- ' Nature,' vol. 55, p. 485.
t ' Phil. Trans.,' A, 1901, vol. 196, p. 525.
