212

They obtained a vigorous effect from Balmain’s luminous paint, but when this was mixed in gelatin there was no external effect.  Schmidt’s results as to the continuance of photo-electric activity when bodies in general are dissolved in each other lead us to believe that an actual conservative property of the medium and not an effect of this on the luminous paint is here involved.  This conservative effect of the gelatin may be concerned with its efficacy as a sensitiser.

In the views I have laid before you I have endeavoured to show that the recent addition to our knowledge of the electron as an entity taking part in many physical and chemical effects should be kept in sight in seeking an explanation of the mode of origin of the latent image.[1]

[1] For a more detailed account of the subject, and some ingenious extensions of the views expressed above, see Photo-Electricity, by H. Stanley Allen:  Longmans, Green & Ca., 1913.

213

PLEOCHROIC HALOES [1]

IT is now well established that a helium atom is expelled from certain of the radioactive elements at the moment of transformation.  The helium atom or alpha ray leaves the transforming atom with a velocity which varies in the different radioactive elements, but which is always very great, attaining as much as 2 x 109 cms. per second; a velocity which, if unchecked, would carry the atom round the earth in less than two seconds.  The alpha ray carries a positive charge of double the ionic amount.

When an alpha ray is discharged from the transforming element into a gaseous medium its velocity is rapidly checked and its energy absorbed.  A certain amount of energy is thus transferred from the transforming atom to the gas.  We recognise this energy in the gas by the altered properties of the latter; chiefly by the fact that it becomes a conductor of electricity.  The mechanism by which this change is effected is in part known.  The atoms of the gas, which appear to be freely penetrated by the alpha ray, are so far dismembered as to yield charged electrons or ions; the atoms remaining charged with an equal and opposite charge.  Such a medium of

[1] Being the Huxley Lecture, delivered at the University of Birmingham on October 30th, 1912.  Bedrock, Jan., 1913.

214

free electric charges becomes a conductor of electricity by convection when an electromotive force is applied.  The gas also acquires other properties in virtue of its ionisation.  Under certain conditions it may acquire chemical activity and new combinations may be formed or existing ones broken up.  When its initial velocity is expended the helium atom gives up its properties as an alpha ray and thenceforth remains possessed of the ordinary varying velocity of thermal agitation.  Bragg and Kleeman and others have investigated the career of the alpha ray when its path or range lies in a gas at ordinary or obtainable conditions of pressure and temperature.  We will review some of the facts ascertained.