Mr. W. Ackroyd on Selective Absorption. 429 



The Five Orders of Spectra. — In the borderland between 

 chemistry and physics, data are being accumulated which tend 

 to make the former a dynamical science. Not the least im- 

 portant of these data are those which furnish us with the con- 

 nexion between the different kinds of spectra. Professor 

 Koscoe is of opinion* that there is not sufficient evidence as 

 yet to support the sequence in the five orders proposed by 

 Lockyerf and now generally received; and in the light of 

 metachromatism it may be necessary to modify this sequence. 

 The juxtaposition of the orders constituting a class of spectra 

 we take to be founded upon that merging of the optical cha- 

 racters of one order into those of another which is exhibited 

 when the molecular changes in the radiating or absorbing body 

 cause such optical alterations. Now confining our attention 

 to low-temperature or absorption spectra, it is evident from 

 what has already been said that change in structural absorp- 

 tion alone is available for forming a sequence of orders. Hence 

 we consider the fifth order, as at present received, misplaced, 

 the unique continuous absorption being from the nature of the 

 experiment a transverse absorptional effect. 



A sequence of orders will readily suggest itself if we keep in 

 mind that change in absorption to which metachromatism is due. 

 Conceive of a colourless body capable of undergoing the whole 

 series of change. It first reflects or transmits rays of every 

 refrangibility; as absorption increases, the band of transmitted 

 or reflected light gradually narrows and becomes less intense; 

 finally the field is darkened and we get continuous absorption. 

 What we propose, then, is shown in the following comparison, 

 made in order that the student may have a clearer conception 

 of the matter. 



Now received. What we propose. 



1st order .... Line spectra \ Class I. 



2nd order .... Channelled-space spectra J Radiative. 



Class II. 



Absorptive. 



1. Continuous absorption. 



3rd order .... Continuous absorption at ] 2. Partial absorption, *. e. ab-t 



the blue end f sorption at the blue and red 



4th order .... Continuous absorption at f .ends, or at any intermediate 



the red end J part of the spectrum. 



5th order .... Unique continuous absorp-1 r< ±- l 



, • u r Continuous spectrum. 



Class II. 1, 2, and 3 constitute the metachromatic scale. To 

 illustrate the bridging over of 1 and 2 no simpler experi- 



* Watts's Dictionary, 2nd Supplement, p. 1093. 

 t Proceedings of the Royal Society, June 11, 1874. 



