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 whose vision they lock up in silence, content with the work of the day."

At the 1903 meeting of the British Association, Kelvin contributed a paper on the subject, of which the following is a résumé:–Radium has been found to emit three types of rays—(1) the α rays, positively electrified and largely stopped by solid, liquid, or gaseous screens; (2) β rays, more penetrative than α and negatively electrified; (3) γ rays, electrically neutral and much more penetrative than either of the other two, passing with but little loss through a lead screen one centimetre thick, which is an almost perfect screen against the other rays. A simple prima facie view was to regard the γ rays as mere vapour of radium; the β rays seem certainly to be atoms of resinous electricity or electrons. The α rays were atoms of molecules of matter, probably atoms of radium, or perhaps molecules of radium bromide. The electro-ethereal hypothesis afforded a ready explanation of the relative penetrating power of the three radiations, and of the fact that each one of them made its existence known to us by conferring electric conductivity on air or on any ordinary gas on which it was present. Taking the γ rays first, we had to explain the free penetration of unelectrified radium molecules through dense liquid and solid matter. An easy assumption sufficed. Let the Boscovichian mutual forces—that was, the chemical affinities and the repulsions—between an atom of radium