Page:Proceedings of the Royal Society of London Vol 69.djvu/434

416 Circle over cell A Opacity log. = 0'59 ; Opacity = 3'89. Circle over cell B Opacity log. = O29 ; Opacity = 1'95.

Ratio B/A = 0-5.

Finally, I tried polonium subnitrate, which gives off emanations hardly capable of passing through any screen, and greatly obstructed by a few centimetres of air.

The apparatus was substantially the same as the one just described, with the modification that the lead cylinder was 12 mm. high, and at the other end a rod of glass 12 mm. high was used to support the film. The reduced height was chosen, experience showing that polonium emanations have great difficulty in penetrating many milli- metres through air. The exposure was 7 days, at the end of which time the film was developed. Over cell A a dark disc sharply defined the inside of the cylinder, while over cell B was a hazy diffused patch which to the eye looked much the fainter of the two. But measurements of patch A and of a disc over cell B of the same size as A, showed that the opacities in each case were practically identical, as shown by the following figures

Circle over cell A Opacity log. = 0*74 ; Opacity = 5'49. Circle over cell B Opacity log. = 0'76 : Opacity = 5'75.

Ratio B/A = 1-05.

A repetition of the experiment, taking the mean of five concordant results, gave the same opacities as before.

Without proving that the emanations from polonium are less material than those from actinium and radium, this experiment shows that their behaviour is entirely different as regards diffusibility through air. Whether this is due to the larger mass of the individual particles, or to the less distance they have to travel (12 mm. as against 28 mm. in the case of actinium and radium), or to some other cause, further experiments must decide.

M. and Mdme. Curie have shown that radio-activity is communicable from radium and actinium compounds to bodies such as lead, copper, glass, ebonite, and paraffin. After exposure to emanations from the radium compound, these bodies have the property of communicating temporary conductivity to the air and other gases, and of thereby discharging an electrified body.

When such charged bodies are exposed to the air, in a single day they lose the greater part of their activity. These phenomena are observed with different radio-active salts of radium, also with salts- containing actinium ; but polonium compounds, even when very active, do not communicate the effect.

Dr. Rutherford shows that air which has remained for some time in the neighbourhood of thoria and then is carried in a current to a