and Dispersion of Light by the Alums. 167 



of the spectrum, it follows that the dispersion -equivalent 



-1 



-j — , or, which is the same thing, 



,^G-/*A 



d ; d ' " ' " "" ~ "7 fe? * d ^ 



of an alum is the sum of the dispersion-equivalents of its 

 constituents. The data by which this can be tested are not 

 so numerous or so trustworthy as in the former case, but the 

 following may be accepted. 



The dispersion-equivalent for water . . . . 0*212 

 „ „ aluminium sulphate 2*40 



„ „ ammonium sulphate 1*33 



„ „ sodium sulphate . 083 



from which may be deduced : — 



Dispersion-equivalent of 



Calculated. 



Observed. 



Ammonium alum 



8-82 

 8-32 



8-40 

 7'65 



Sodium alum 





Though these figures are tolerably accordant, it will be seen 

 that those in the first column are decidedly higher than those 

 deduced from Soret's measurements. The differences are about 

 5 and 8 per cent, respectively ; but there are known sources 

 of error in experiment which may affect the first place of 

 decimals. 



The dispersion-equivalents of the different alums may be 

 thus tabulated : — 





Dispersion-equivalents of the Alums. 



Alumi- 

 nium. 



Chromium. 



Iron. 



Indium. 



Gallium. 



Ammonium salt ... 



8-40 



991 



1211 







Sodium „ 



765 











Methylamine „ 



8-47 











Potassium „ 



7-92 



973 



11-31 







Kubidium „ 



8-36 



9-87 



11-86 



8-98 



8-73 



Caesium „ 



8-44 



1015 



12-28 







Thallium 



10-98 



1213 



1445 







It is evident at once : — 



First. That the differences due to the replacement of one 

 metal by another are very considerable ; more considerable in 

 proportion than the differences in the case of the refraction- 

 equivalents. 



Secondly. That the different compounds of the alkalis pre- 



