68. TRANSMISSION OF EXTERNAL CATHODE RAYS THROUGH METALS.—The phosphorescence was not diminished apparently by an intervening gold-leaf or silver or aluminum foil, while it was extinguished by quartz .5 mm. thick which also cut off the atmospheric glow beyond itself. The leaves and foil did not so act. The difference of thickness should be borne in mind, as metal, as thick as the quartz did not transmit. As to other substances, tissue paper cast a slight shadow, which was darker with an additional sheet; but the shadow was independent of color and blackness, § 154. Ordinary writing paper was roughly, proportionally opaque, while the shadow was black with cardboard .3 mm. thick. Glass films as made by blowing glass, cast faint shadows when .01 mm. thick. He proved that there was little difference as to the transmitting power of conductors and dielectrics when thin. Mica and collodion sheets .01 mm. thick cast scarcely any shadow. The reader may bear in mind the striking differences between these properties of cathode rays, and X-rays, § 135, it being assumed always that the generating devices are the same; for example, water permitted the cathode rays (were these simply feeble X-rays?) to be transmitted only when in very thin layers. Even soap water films which were only .0012 mm. thick cast shadows, although very faintly. The shadows of drops of water were black, while water several feet thick has been traversed by X-rays from a small set of apparatus. By careful measurements he found that the law of transmission must be different from that of light, for in the latter, many substances are opaque although exceedingly thin, while with cathode rays, the same will traverse all films. Goldstein and Crookes reported that thin mica, glass and collodion films made very dark shadows, § 58, within the discharge tube, whereas Lenard found that outside of the vacuum tube, in open air, the transparency was greater than according to the earlier experimenters, but he acknowledged that Crookes and Goldstein were inconvenienced and limited in the number of observations because it is so difficult to carry on such experiments within an hermetically sealed tube. Again, he acknowledged that perhaps the cathode rays of those experimenters were of a different kind. The construction shown in the above figures was modified by using a very thin glass window instead of aluminum, and the results were the same allowing for the different opacity, to ordinary light, of aluminum and glass. The cathode rays acted upon the sense of smell and taste as the nose and mouth could detect ozone, § 84, at end.