292 SCIENCE PROGRESS 



molecule of HBr is only eight points higher than that of one 

 atom of bromine, so that if the whole change in refractivity 

 could be debited to the hydrogen atom, it could be said that 

 eight-ninths of its refractivity have disappeared. But, turning 

 to HI, a greater surprise is in store. The refractivity of a 

 molecule of hydriodic acid is actually less than that of an atom 

 of iodine alone. To what cause is this due ? The answer 

 would perhaps supply us with the clue to the nature of chemical 

 combination. On the assumption that refractivity is due to the 

 action of the electrons which enter into the composition of an 

 atom, it is a plausible guess that in combination one or more 

 electrons jump across from one atom to the other, and that in 

 its new position it is in a field of force where it does not exert 

 so great a retarding effect. Such a theory has an attractive 

 similarity to the conceptions of electrolytic theory. It has 

 actually been put forward as a possibility by Sir William 

 Ramsay and others, and it is worthy of further examination. 



The tendency to the reduction of refractivity shown in the 

 cases above mentioned is not without parallel. In 1906 the 

 refractivities of the hexafluorides of sulphur, selenium, and 

 tellurium were determined in the gaseous state. The purity 

 of the compounds is guaranteed by the tests to which Dr. 

 E. B. R. Prideaux 1 subjected them. 



The results show a large reduction of refractivity on combina- 

 tion in all three cases. A molecule of SF G shows a reduction equal 

 to more than half the refractivity of a sulphur atom. A molecule 

 of SeF 6 retards light very little more than an atom of selenium, 

 while a molecule of TeF 6 retards it by a good deal less than an 

 atom of tellurium alone. In this case retarding power equal 

 to the six fluorine atoms and one quarter of the tellurium atom 

 had disappeared. Though no explanation of these facts can 

 be offered, it is suggestive to reflect that the great chemical 

 activity of fluorine is accompanied by the power of forming 

 compounds of low refractivity. One very interesting conclusion 

 can be drawn from the two series. The case of TeF 6 shows 

 that on the hypothesis that the six fluorine atoms lose refractive 

 power by losing an electron each, the tellurium atom must also 

 lose refractivity in gaining six electrons, and any theory which 

 seeks to explain the nature of refractivity must take that 



1 " Some Reactions and New Compounds of Fluorine," by E. B. R. Prideaux 

 {Trans. Chem. Soc. 1906, vol. 89, p. 316). 



