l;| ;i -I: ACTION ANI> IHSI'KKSIOX OF TIIK HALOGENS, HALOGEN ACIDS, ETC. 5 



After each experiment was over tl..- -as was iiUirlied over soda lime and only 

 experiments in which the in.|..iriti.-s were negligible were used in determining the 

 refractive index, 



The light used WM that ..f tin- green mercury line, A = 



Refractivity. The following figures were obtained in fire experiments, the 

 experimental values U-iiig reduced to <>' < '. ami 7C,n nun. liy the formula :- 



^ T 760 



-J73-T-' 



Ou-OlO 7 . . . . 797i, 7UK5, 7'J6. 7'JKo, 7981. Mean 7980. 



St.n.'i'ir'l ( 'onditiona. 'I'he practice of reducing observations of refractivity to the 

 standard temperature of .1 < '. and the pressure of 760 mm., dates from a time when 

 deviations fn-m the laws of BOYI.K and GAY-Lrss.vr were alike unknown. As 

 accuracy improved and the field of research was extended to vapours, these ,-///. 

 became insufficient and sometimes meaningless. MASCART, the volume of whose 

 work entitles him to l>e considered the leading authority on the subject, at first 

 adopted the old conditions, and even in the OBM of SO, expressed the refractivity as 

 it would lie at and 760 mm. But, in his later work, when dealing with chlorine 

 ..,.! bromine and with some organic compounds f,,r which the coefficients of thermal 

 expansion and compressibility were unknown, be contented himself with determining 

 the refractivity at pressures as low as possible and comparing it with that of air at 

 the same temperature (12 C).* 



LK Roux, in bis experiments on sulphur, mercury, phosphorus and arsenic 

 expressed the ratio of the refractivity to the density, and LORENZ and PRYTZ adopted 

 the same system. It is evident that this principle is the most convenient for those 

 who wish to compare the refractivity of equal numbers of atoms of different elements, 

 or of molecules of different eomixmnds. Accordingly, in the present work, we have 

 reduced all ref inactivities to the values which they would have had if the gas or 

 vapour had the density of hydrogen at C. and 76 cm., ('000089849) gr./(cm.)- 

 multiplied by the ratio of the theoretical molecular weight of the substance in 

 question to that of hydrogen. But, in order to avoid confusion, we shall denote this 

 value by the syinlx>l ( M -l) rP^r , where D denotes the standard density as here 



defined, and (<!-,,,} the density at ()" C. and 7i cm 



The density of chlorine at the temperature and pressure of t he at mospliere has 

 determined recently by TI;I: M.WKI.I. and CnRI8TIE.t They found that at "J(f C., the 

 molecular volume Wfl L'-jn:!-.! and liUOaO'O at 10 C. 



* MASCAKT, 'C. R.,' v.,1. s6, pp. :ii!l and lls-j. 



t F. P. TRKALWELL and W. A. K. CHKISTIK, 'Zcits. aiiorg. Chem.,' Vol. 47, p. 446, 19( 



