The Curies at once determined that the minute quantity of colourless crystals they had obtained was the chloride of a new metallic element with the atomic weight 225, to which they gave the name radium. The proof that radium is an element is given by its ‘sign-manual’--the spectrum which it shows to the observer when in the incandescent state. It consists of six bright lines and three fainter lines in the visible part of the spectrum, and of three very intense lines in the ultra-violet (invisible) part (fig. 10). A very minute quantity is enough for this observation; the lines given by radium are caused by no other known element in heaven or earth. They prove its title to be entered on the roll-call of elements. [Illustration: FIG. 10. A diagram of the _visible_ lines of the spectrum of the elements Radium and Helium--when rendered incandescent by electric ‘sparking’ in a glass tube: kindly prepared for this book by Mr. Frederick Soddy of the University of Glasgow. The position of the chief great lines of the solar spectrum are marked on the lowest horizontal line. On the upper line the wave-lengths of the rays occupying the position indicated, are given. The figure 72 means that the wave-length of the ray occupying this position when refracted by the prism of the spectroscope is, as measured from crest to crest of the undulation, seven hundred and twenty millionths of a millimetre. It is generally written 720·0 µµ. Lines exist at the ultra-violet end of the spectrum which can be photographed but do not affect the eye--that is to say are invisible. On the other hand the lines of the red end of the spectrum do not produce a photographic effect. Consequently a ‘_photographed_’ spectrum such as that given in the next figure (fig. 11) differs in the lines presented both at the red and the violet ends from the _visible_ series of lines. The two (visible and photographed spectra) agree only from wave-length 587·6 µµ to wave-length 447·2 µµ. The two spectra given in fig. 10 show how great is the difference in the position and number of the bands of Radium and Helium--yet as shown in the next figure (fig. 11) the ‘emanation’ from Radium actually is transformed into Helium.]