420 Sir W. Crookes. 



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. 



