394 PROTOPLASM 



vacuum tube are of particular interest, though undoubtedly 

 the other forms of emanation from such tubes have their effect 

 on organisms. Cathode rays have already been used in biologi- 

 cal experiments. 



Passing on toward that end of the spectrum with shorter 

 waves, we come to the gamma rays which overlap the X rays. 

 Gamma rays are emitted by radium. They are more penetrat- 

 ing than X rays and have the property, like the latter, of ionizing 

 (charging) air. Gamma rays will penetrate a foot of water and 

 still have half of their intensity left, while X rays are absorbed 

 by an inch of water. 



The wave spectrum ends with cosmic rays, the shortest 

 known— so short that were they increased in length until they 

 equaled the thickness of a card — then the longest wireless wave, 

 if enlarged in the same proportion, would extend from the earth 

 to the nearest star. Cosmic rays have come into considerable 

 prominence of late. Their presence in the atmosphere was first 

 suspected by Rutherford. He thought them due to the radio- 

 active materials in the earth. At that time, they were referred 

 to as the "penetrating radiation" of the atmosphere. When it 

 was found that this radiation did not decrease so rapidly as it 

 should with altitude, the German physicist Gockel took an elec- 

 troscope up in a balloon to a height of 13,000 ft. and found that 

 the penetrating radiation still did not decrease. Higher altitudes 

 (5.6 miles) revealed a radiation several times as great as that at 

 the earth's surface, which indicated that the radiation must be of 

 cosmic origin. Milhkan then continued the work and measured 

 the intensity of the rays in a mountain lake to a depth of 45 ft. 

 The atmosphere above the lake was equivalent in absorbing 

 power to 23 ft. of water, so that cosmic radiation can penetrate 

 45 plus 23 or 68 ft. of water, the equivalent of 6 ft. of lead. 

 Cosmic rays are "harder," more penetrating, than any others 

 known. 



Some interesting speculation has been done on the possibility 

 that the source of cosmic rays is the energy lost when hydrogen 

 unites to form helium. Millikan regarded the presence of 

 cosmic radiation as direct evidence of atom building out in the 

 depths of interstellar space. The atomic weight of hydrogen 

 is 1.0078; the mass of the helium atom is 4; therefore, when four 

 hydrogen atoms unite to form one helium atom, there is a loss 



