424 Lord Rayleigh on Dynamical Problems in 



however, like those of chromic oxide and the oxide of zinc, 

 in which a shifting of a region of maximum of reflecting- 

 power towards the longer wave-lengths actually occurs when 

 the pigment is heated. 



It had been our purpose in the investigation just described 

 to extend our spectro-photometric measurements to the very 

 low temperatures obtainable by the use of solid carbon 

 dioxide, but we have been compelled by lack of time to content 

 ourselves with noting such changes as could be detected with 

 the unaided eye. The change observed was in every case 

 that which would be brought about by increase of reflecting- 

 power. There was no increase in the saturation of the 

 colour, rather, on the other hand, a paling or dilution of the 

 tint, as though there were a tendency towards white. Houston*, 

 who made a similar set of observations at the higher tempera- 

 tures reached by the evaporation of carbon bisulphide or 

 sulphurous acid, arrived at a similar result. Ackroyd, from 

 theoretical considerations, concluded that as the absolute zero 

 is approached the prevailing tints of pigments will be blues 

 and violets, merging finally into white. 



Ackroyd, Hartley, as also Houston and Thomson, and still 

 earlier Schoenbein and Brewster, have had something to say 

 concerning the explanation of these phenomena. Their 

 various views need not be touched upon here, unless it be to 

 call attention to the opening paragraph of Ackroyd's paper, 

 which contains an important statement. Ackroyd says : — 

 " These changes embrace a class of phenomena, quite as 

 important in their way as 9 phosphorescence and fluorescence, 

 with which in fact they are intimately connected." It is our 

 opinion that the connexion is indeed a most intimate one, and 

 that every change of colour that pigments undergo is to be 

 regarded simply as a symptom of changes in the radiating- 

 power of the substance. 



Physical Laboratory of Cornell University, 

 June 1891. 



LIU. Dynamical Problems in Illustration of the Theory of 

 Gases. By Lord Kayleigh, Sec. R.S.^ 



Introduction. 



rT^HE investigations, of which a part is here presented, had 

 1 their origin in a conviction that the present rather un- 

 satisfactory position of the Theory of Gases is due in some 



* Loc. cit. p. 123. 



t Communicated by the Author. 



