404 



Dr. J. H. Gladstone. 



[June 16 r 



dispersion ; and that, on the ' other hand, eymol and hexahydro- 

 naphthalin, both having the formula C 10 H 14 , but the first three pair, 

 and the second two pair of doubly-linked carbon-atoms, have nearly 

 the same dispersion. But if we reckon out the refraction equivalents 

 for cinnamic aldehyde and for anethol from the numbers given in the 

 same table, it will be seen that they are inconsistent with the sup- 

 position that these bodies have the chemical structure that he- 

 attributes to them ; in fact the extremely high dispersion in each case 

 only tells the same tale as the extremely high refraction. As to the 

 two substances of the formula C 10 H 14 it is open to question whether 

 hexahydronaphthalin has only two pair of doubly-linked carbon- 

 atoms; and the refraction equivalent calculated for each of the 

 specimens throws some doubt upon their purity. Briihl also com- 

 pares methyldiphenylamine with cinnamic aldehyde, but the presence- 

 of nitrogen in the first body, and the uncertainty as to the constitu- 

 tion of the second, render it unsafe to draw any conclusions from the 

 comparison. That the specific dispersion of isomeric or polymeric 

 bodies is practically the same, except where the constitution is very 

 different (as in aniline and picoline), was shown in my paper in the 

 ' Philosophical Magazine ' six years ago ; and this must be set against 

 the doubtful cases mentioned above. 



The Elements. 



There are but few of the elements of which the dispersive energy 

 can be directly determined ; but it so happens that two or three of 

 these are among the most dispersive of bodies. 



Phosphorus was determined by Mr. Dale and myself in a melted 

 condition, and also by Damien both in that and the solid state. Our 

 observation gives 3"1 ; those of Damien* work out at 2*9 and 2'8 

 respectively. 



Sulphur. — An old observation of mine on this body liquefied, gave 

 0'90 for E— A; and recent observations from its solutions in bisul- 

 phide of carbon give 1*2 for F— A. These agree in indicating about 

 2'6forH-A. 



Selenium. — According to the observations of Sirks,f the refractive 

 indices for A and D are respectively 2*653 and 2*98; taking the specific 

 gravity at 4*5, the dispersion equivalent of this element would be the 

 extraordinary amount of 5*67 for D— A alone. 



Hydrogen. — Ketteler's J observations give a dispersion equivalent 

 of 0"0152 for the difference between the green line of thallium and 

 the red line of lithium. 



# 'Journal de Physique,' 1881. 



t ' Poggendorff, Annalen,' vol. 143, 1871, p. 429. 



X ' Poggendorff, Annalen,' vol. 124, 1865, p. 390. 



