CHAPTER XXI. 



SOME TEST EXPERIMENTS. TRIAL OF NEW METHODS. 



THERE was still however a difficulty in many minds in as- 

 cribing changes in spectra to temperature alone, and therefore 

 to a possible dissociation. It thus appeared to be one's duty to 

 find an experimentum crucis if possible. 



It seemed as if the results of experiments based on the 

 following considerations ought to be accepted as throwing light 

 on the question. 



First Consideration. 



At a low temperature some substances give us few lines while 

 at a high one they give us many. Vapours, therefore, already 

 glowing with few lines at a low temperature, say in a flame, 

 should give us all their lines when the vapour is suddenly sub- 

 jected to a high one, say by the passage of a high tension spark. 

 On the bell hypothesis the spectrum should change with the 

 mode of striking. On the dissociation hypothesis this should 

 only happen for the lines of those molecular groupings which 

 are from other considerations held to be more simple. If the 

 flame has brought the substance to its lowest state, the passage 

 of the most powerful spark should not cause the flame spectrum 

 to vary. 



Now what are the " other considerations " above referred to ? 

 This necessitates a slight digression. 



Of the short lines (see ante, p. 213), the explanation generally 



