344 Dr. Ketteler on the Dispersion of Light in Gases. 



millira. 



fAir ....... . X = 0*0000723 



For -j Hydrogen =0-0000831 



[Cyanogen =0*0000958 



/Water =0*0000813 



" \Sulphide of carbon . . . =0*0001235 



„ Heavy glass =0*0000959 



The idea consequently easily suggested itself that this quan- 

 tity might remain strictly constant for the passage of a substance 

 from one state of aggregation to another, especially as X appears 

 as the wave-length in a vacuum. I have tested this conclusion 

 in the case of sulphide of carbon, and, particularly, in that of sul- 

 phurous acid. 



After encountering many difficulties, the ratio of the numbers 

 of simultaneously displaced red and yellow bands was found to 



45*5 



be, for sulphide-of-carbon vapour = ^-p ; while the ratio calcu- 



lated, upon the above supposition, from the constants applicable 



to liquid sulnhide of carbon is = or . * 



6\)do 



Liquid sulphurous acid was also submitted to careful measure- 

 ment. The indices obtained were 



7z Li = 1*33574 • n m = 1*33835 ; n Tl = 1*34108. 



For the combination of rc Li and w T1 , formula (III) gives 



A = 0*00009404 millim., « = 0*32692, 

 and hence 



nVa-=I'33838j 



and therefore, in units of the fifth place, n— n'= — 3. 



The temperature was 24°*1 C. Now for liquid sulphurous acid 

 we have 



^=1*4821 (Pierre), A=~ =0*221, X = 0*0000940 millim.; 



for gaseous sulphurous acid, 

 d=2*2i .6. 0-001293 (Dulong), A = 0*225, X =0*0000981. 



It will be seen that the two constants remain exactly identical 

 even within these exceedingly wide limits. 



In the introduction to this paper I insisted on the importance 

 of the study of the optical properties of gaseous bodies, and de- 

 clared the method which I have followed to be the only appro- 

 priate one. The experiments that have been recorded will, I 

 think, justify this assertion ; for the method employed has ren- 

 dered possible the first successful measurements of the delicate 

 phenomenon of the chromatic dispersion of light in gases, and 



