236 Royal Society. 



spectra at those temperatures, although we undoubtedly get line- 

 spectra when electricity is employed, as stated in 4) give us a con- 

 tinuous spectrum at the more refrangible end, the less refrangible 

 end being unaffected. 



(y) At ordinary temperatures, in some cases, as in selenium, the 

 more refrangible end is absorbed ; in others the continuous spec- 

 trum in the blue is accompanied by a continuous spectrum in 

 the red. On the application of heat, the spectrum in the red dis- 

 appears, that in the blue remains ; and further, as Faraday has 

 shown in his researches on gold-leaf, the masses which absorb in 

 the blue may be isolated from those which absorb in the red. It 

 is well known that many substances known to be compounds in so- 

 lution give us absorption in the blue or blue and red ; and, also, 

 that the addition of a substance known to be compound (such as 

 water) to substances known to be compound which absorb the 

 blue, superadds an absorption in the red. 



7. In those cases which do not conform to what has been stated 

 the limited range of the visible spectrum must be borne in mind. 

 Thus I have little doubt that the simple gases, at the ordinary con- 

 ditions of temperature and pressure, have an absorption in the 

 ultra-violet, and that highly compound vapours are often colourless 

 because their absorption is beyond the red, with or without an ab- 

 sorption in the ultra-violet. Glass is a good case in point ; others 

 will certainly suggest themselves as opposed to the opacity of the 

 metals. 



8. If we assume, in accordance with what has been stated, that 

 the various spectra to which I have referred are really due to dif- 

 ferent molecular aggregations, we shall have the following series, 

 going from the more simple to the more complex : — 



First stage of complexity 1 L; ectrum- 



or molecule J r 



Second stage Channelled space-spectrum. 



f Continuous absorption at the blue 

 end not reaching to the less refran- 

 gible end. (This absorption may 

 breakup into channelled spaces.) 

 Continuous absorption at the red end 

 not reaching to the more refran- 

 gible end. (This absorption may 

 break up into channelled spaces.) 

 Fifth stage Unique continuous absorption. 



9. I shall content myself in the present note by giving one or 

 two instances of the passage of spectra from one stage to another, 

 beginning at the fifth stage. 



From 5 to 4. 

 1. The absorption of the vapours of K in the red-hot tube, de- 

 scribed in another note, is at first continuous. As the action of 

 the heat is continued, this continuous spectrum breaks in the 

 middle ; one part of it retreats to the blue, the other to the red. 



Third stage - 



Fourth stage 



