PREFACE. xi 



by the employment of moderate temperatures moderate, that 

 is to say, in comparison with the transcendental dissociating 

 energies of electricity available in our modern voltaic arcs 

 and electric sparks. 



But, in the observations made during the last thirty years 

 on the spectra of bodies rendered incandescent by electricity, 

 we have actually, though yet scarcely consciously } been employing 

 these transcendental temperatures ; and if it be that this higher 

 grade of temperature does what all other lower grades have 

 done, then the spectrum we have observed in each case is not 

 the record of the vibrations of the particular substance which 

 we have put into the arc and with which we have imagined 

 ourselves to be working alone, but of all the simpler substances 

 or forms of the same substance produced by the short or long 

 series of the " separations " effected. 



The question, then, it will be seen, is an appeal to the law 

 of continuity, nothing more and nothing less. Is a tempera- 

 ture higher than any yet applied to act in the same way as 

 each higher temperature which has hitherto been applied 

 has done ? Or is there to be some unexplained break in the 

 uniformity of nature's processes ? 



I give in the present volume the reasons which compel me to 

 hold that the answer to the question put is, that these tran- 

 scendental temperatures do dissociate, and that therefore what 

 has hitherto been taken for granted is, in all probability, not 

 true, and I attempt to show that both in observatory and labora- 

 tory the spectroscopic phenomena observed are simply and 

 sufficiently explained on the view that the so-called chemical 

 elements behave after the manner of compound bodies. 



