The alpha rays have little penetrating power, a sheet of paper or a sheet of aluminum 0.05 mm. stopping them. Upon losing their charges they become atoms of helium. Their velocity is about 1/10 of that of light or 18,000 miles a second. The _spinthariscope_ is a little instrument devised by Sir Williams Crookes in 1903 to show direct evidence that particles are continually being shot off from radium. In this instrument (Fig. 424), a speck of radium _R_ is placed on the under side of a wire placed a few millimeters above a screen _S_ covered with crystals of zinc sulphide. Looking in the dark at this screen through the lens _L_, a continuous succession of sparks is seen like a swarm of fireflies on a warm summer night. Each flash is due to an alpha particle striking the screen. The beta rays are supposed to be cathode rays or electrons with velocities of from 40,000 to 170,000 miles a second. The gamma rays are supposed to be "X" rays produced by the beta rays striking solid objects. [Illustration: FIG. 424.--A spinthariscope.] =424. The discovery of radio-activity= has revolutionized the ideas of the constitution of matter. Further, the results of experiments upon radio-active materials reveals the presence of immense quantities of sub-atomic energy. If man ever discovers a means of utilizing this, he will enter a storehouse of energy of far greater extent and value than any of which he has as yet made use. A consideration of this unexplored region gives zest to the work of those who day by day are striving to understand and control forces of nature. Important Topics 1. Oscillatory nature of discharge of Leyden jar. Proofs. 2. Wireless telegraphy and telephony. 3. Electrical discharges in rarefied gases. 4. Cathode and "X" rays. 5. Electromagnetic theory of light. 6. Radio activity and radium. [Illustration: CONTINENTAL TELEGRAPH CODE A . - J . - - - S . . . B - . . . K - . - T - C - . - . L . - . . U . . - D - . . M - - V . . . - E . N - . W . - - F . . - . O - - - X - . . - G - - . P .- - . Y - . - - H . . . . Q - - . - Z - - . . I . . R . - . PERIOD INTERROGATION EXCLAMATION . . . . . . . . - - . . - - . . - - 1 . - - - - 2 . . - - - 3 . . . - - 4 . . . . - 5 . . . . . 6 - . . . . 7 - - . . . 8 - - - . . 9 - - - - . 0 - - - - -] CHAPTER XVIII WIRELESS TELEPHONY AND ALTERNATING CURRENTS The developments in wireless communication have been so rapid during recent years that a more extended account, than that given in Art. 417 of the apparatus and methods used at the present time, seems desirable. The study of Alternating Currents is also included with the idea that it will make the text more complete and of wider usefulness. WIRELESS TELEPHONY