140 BEPORT— 1882. 



DOW that, thanks to Captaiu Abney, we may photograph in the ultra-red, 

 we may test its truth ; but we must observe that, if we once allow our- 

 selves to take these harmonics to aid, we may arrange all our spectra 

 in any order we like, for we need only assume that those which do not 

 fit are higher or lower harmonics of the true spectrum. As they stand 

 the spectra of the chlorides of barium, strontium, and calcium, though 

 showing certain characteristic analogies, do not bear out Lecoq's theory. 



Ditte ' has tried to find an analogy between the spectra of sulphur, 

 selenium, and tellurium, and to establish that the spectrum is displaced 

 towards the blue as we go from the metalloid to the metal, that is to say, 

 from the lighter to the heavier element. An inspection of the spectra in 

 question has not led the author of this report to confirm this statement. 

 The spectrum of selenium seems more contracted than the spectrum of 

 sulphur, but we cannot recognise clearly any displacement towards the 

 red or towards the blue. The spectrum of tellurium seems, if anything, 

 more to the red than that of the other two metals. 



Messrs. Troost and Hautefeuille ^ have compared the spectra of 

 carbon, boron, silicium, titanium, and zirconium, and they also come 

 to the conclusion that from the metalloid to the metal the spectrum seems 

 progressively to move towards the blue. 



As far as the fii-st three bodies are concerned the relation seems, at 

 first sight at any rate, to have some foundation, for we have here to deal 

 with three spectra which, on the whole, resemble each other in appear- 

 ance, and which seem to be displaced according to the suggested law. 



Ciamician ^ has compared together the spectra of chlorine, bromine, 

 and iodine. His experiments seem well conducted, and we therefore give 

 his conclusions, reserving, however, every opinion as to the degree of 

 certainty with which they have been established. It requires a much 

 more careful experimental examination than even Ciamician has given 

 to them to arrive at any proof of the reality of these analogies. The 

 spectra in question are so variable with density and temperature that we 

 cannot at present say whether we have to deal with a superposition of 

 different spectra or simply a variation of relative intensity due to tem- 

 perature. Some of Ciamician's analogies do not seem, certainly, to be 

 very apparent, but the following conclusion is entitled to a place in this 

 report :^ 



' The spectrum of vapour of bromine at low pressures becomes the 

 more nearly like that of chlorine the smaller the pressure, while the 

 spectrum of condensed bromine more nearly resembles the spectrum of 

 iodine. Iodine, on the other hand, gives a spectrum resembling that of 

 bromine at moderate pressures only ; at very low pressures an analogy 

 with the chlorine spectrum takes preponderance over that with the 

 bromine spectrum. The spectrum of highly condensed iodine vapour 

 cannot well be compared with that of the other two bodies. Chlorine, 

 when it is highly condensed, gives a spectrum resembling that of bromine 

 at large pressures and of iodine at moderate pressures ; while at low 

 pressures the spectrum of chlorine has no analogy to that of the other 

 two elements. 



' If we collect together for each body all the lines which appear 

 separately under different circumstances, we can establish a complete 

 correspondence between the lines of the three complete spectra.' 

 • C. R. Ixxii. p. 622 (1871). ^ (j_ j}^ ixxii. p. 620 (1871). 



■' Wie/i. Ber. Ixxviii. (1878). 



