MESSRS. C. CUTHBERTSON AND E. PARR METCALFK 



The results are very curious. GLADSTONE'S refraction equivalent for arsenic* is 

 about twelve times that for hydrogen, while the gaseous refractivity is eleven times 

 that of hydrogen. But the values of GLADSTONE for zinc and cadmium are just half 

 those now obtained for the gaseous state. HAAGEN, however, has a value for zinc 

 which corresponds fairly well with the refractivity. Finally, GLADSTONE'S value of 

 selenium is about double that shown by the refractivity. 



Relative Refi-actimties of the Elements. 



Previous work in this field had brought to light certain relations between the 

 refractivities of the elements which seemed too regular to be due to chance, and the 

 present work was undertaken mainly with the object of ascertaining whether similar 

 relations existed in the case of other elements. 



The results have been partly successful and parti}' unsuccessful. It was at once 

 manifest that, in view of the great dispersion and in the absence of trustworthy 

 values for infinitely long wave-lengths, the search for simple integral ratios between 

 the refractivities must be postponed. But in one group of elements traces of a 

 tendency to conform to such a rule may, we think, be fairly claimed. 



Table VIII. shows the elements arranged in the order of their atomic weights, with 

 the refractivities appended in those cases in which they have been measured in the 

 gaseous state for the D line. The form of the table is suggested by the ratios 

 existing between the refractivities of some of the elements, and is, so far as we know, 

 somewhat different from previous forms of the periodic table. 



Thus, since the refractivities of nitrogen, oxygen, fluorine and neon are respectively 

 one fourth of those of phosphorus, sulphur, chlorine and argon, it would seem that 

 each of these groups should form a horizontal row, and since the refraction equivalents 

 of potassium, rubidium and caesium are in the same ratio as those of argon, krypton, 



* In this connection it is interesting to compare the values now found with those suggested by the 

 indices of some compounds of the elements. 



As an instance we may take the case of arsenic trichloride, whose index (as found by HAAGEN) is 

 approximately 1'6. Converting this number into the corresponding one for a gas by the formula of 



LORENZ, ^- = constant, we arrive at the figure 1920; of this 1152 may be subtracted for the 



ft- + 2 a 



chlorine atoms, leaving a Iwlance 768 for one atom of arsenic, or 1536 for two atoms of arsenic, a result 

 which corresponds well with the number 1550 now found. But the additive rule, as is well known, is not 

 of universal application. It fails conspicuously in the case of the fluorine compounds of the sulphur, 

 selenium, tellurium group. The present writers were, through the courtesy of Dr. E. B. R. PKIDEAVX, 

 afforded the opportunity of measuring the refractivities of these compounds. The results have been 

 published already in Dr. PRIDEAUX' paper in ' Trans. Chem. Soc.,' 1906, vol. 89, p. 330. For the 

 refractivities of the hexafluorides of sulphur, selenium, and tellurium we found the values 783, 895, and 

 991, the corresponding numbers deduced by the additive rule being 1116, 1356, and 1826 respectively. 



