. The Becquerel rays and the properties of radium . know how many atoms there are in a gramme of hydrogen, or,whalj comes to the same thing, how many in a cubic centimetre. This canbe calculated in various ways ; but they are all rather indirect, and theresults are not as concordant as might be wished. Still, the number iscertain within a few times. B 18 THE BECQUEREL BAYS celebrated discoverer of electro-magnetic waves. Hefound that if a film of gold leaf was placed in thepath of the cathode rays, they were able, notwith-standing, to produce some phosphorescence on theglass wall of the tube,
. The Becquerel rays and the properties of radium . know how many atoms there are in a gramme of hydrogen, or,whalj comes to the same thing, how many in a cubic centimetre. This canbe calculated in various ways ; but they are all rather indirect, and theresults are not as concordant as might be wished. Still, the number iscertain within a few times. B 18 THE BECQUEREL BAYS celebrated discoverer of electro-magnetic waves. Hefound that if a film of gold leaf was placed in thepath of the cathode rays, they were able, notwith-standing, to produce some phosphorescence on theglass wall of the tube, though the intensity of thephosphorescence was greatly reduced by passageof the rays through the leaf. Lenard went a step further. He arranged a smallwindow of thin aluminium foil at the end of a vacuumtube. This window was only about ^^ inch indiameter ; the object of making it so small was tosupport the thin leaf as far as possible against theatmospheric pressure, which tends to burst it this arrangement (fig. 7) Lenard was able to. Fio. 7.—Tube for showing penetration of the cathode rays into the open, a is thedisc-shaped catliode ; a metal tube, b, serves as anode. Opposite the cathode is the thinaluminium window, o, carried by the brass cap, d. The rays issue from this window. get the cathode rays through the window and outinto the open air. This experiment does not succeedunless the vacuum tube is very highly exhausted, soas to obtain rays of great velocity. The rays of smallvelocity, obtained at higher pressures, are unable topenetrate the window. Lenards experiments, at thetime when they were first published, were consideredby many to constitute a fatal objection to thecorpuscular theory of the rays. It was argued thatcorpuscles could not penetrate a solid metal window,however thin. This is no longer felt to be a serious ELECTEIC DISCHAKGE IN HIGH VACUA 19 difficulty, as we shall see when we come to discussmodern ideas of the constitution of matter. The c
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