THE TRANSFORMATION OF ELEMENTS 223 



temperature, and the hydrogen and any helium pumped off 

 (see p. 75). After a series of measurements had been made 

 during twenty-eight days, a scarcely visible amount of gas 

 remained. On passing a spark and freezing out the mercury 

 vapour, the helium spectrum was visible. 



This experiment proved conclusively that helium is a product 

 of the disintegration of radium emanation. Whether it is 

 produced directly from radium itself can only be ascertained 

 when the relative volumes produced from radium and emanation 

 together, and from emanation alone, have been accurately 

 determined. 



The change of radium emanation into helium has been 

 confirmed by numerous observers. Since this is perhaps the 

 most striking of all the transformations, some account is given 

 of their work. 



In nearly all cases the spectrum was observed with a direct 

 vision spectroscope. The eye-piece was adjustable by means of 

 a micrometer screw, which could be read to the hundredth part 

 of a millimetre. The spectroscope was either standardised 

 previously by observing a series of known spectra, or else the 

 spectrum of the suspected gas could be observed simultaneously 

 by means of a comparison prism. In the former case a con- 

 venient method is to use a tube containing hydrogen, helium, 

 and mercury vapour, which give a number of strong lines 

 throughout the visible spectrum. By plotting the measured 

 screw-readings of these lines against their known wave-lengths, 

 a curve is obtained, from which the wave-length corresponding 

 to any particular measurement can be determined accurately to 

 one or two Angstrom units. 



Curie and Dewar in 1904 confirmed the production of 

 helium in the following manner : About 0*4 gram of radium 

 bromide was placed in a small quartz bulb joined to a quartz 

 tube, and the whole connected with a mercury pump. After 

 evacuation the radium salt was heated till fusion and the whole 

 again evacuated. The quartz bulb and tube were then sealed 

 off. Twenty days later Deslandres examined the spectrum of 

 the gas contained in the tube, using external electrodes. He 

 observed the entire spectrum of helium, and no other lines, 

 although a current was passed through the tube for three 

 hours. 



Indrikson carried out an experiment with the gases from 



