i;i.n;\(TIVF. INDICKS OF IMF. F.I.F.MF.NT.v 



341 



On comparing the new value with that of nitrogen, the first member of the 

 chemical group to which phosphorus belongs, a very remarkable relation is at once 

 apparent. The refractive index of N* for sodium light has been determined by 

 MASCART to be 1 -0002973 and by KAMSA.Y and TRAVERS 1-0002971. Accepting the 

 mean of these observations, it will be seen that the ratio of the refractivity of P, 

 to that of N 4 is 4'03 ; or, in other words, it appears that, to an accuracy of 1 per 

 cent, which is within the limits of experimental error, an atom of phosphorus retards 

 liy fit four times as much as an atom of nitrogen. 



Sulphur. (Atomic Weight = 32'06.) 



The study of the refractive index of sulphur presents comparatively little difficulty, 

 for it is easily obtained pure, and can be weighed and introduced into the tube with 

 facility. 



Sulphur, well washed in distilled water to free it from traces of acid, and dried at 

 100 C'., was redistilled in r<icno. The distillate was melted and sucked into capillary 

 tul>es, forming thin sticks whence a small cylinder of the proper weight was broken 

 oft". This could l)e conveyed into the tulje without loss of weight. 



Two experiments were found necessary to discover the maximum quantity of 

 sulphur which would be vapourised at the highest temperature available. When this 

 had been done, the results obtained were so concordant that only four experiments 

 were necessary to give a trustworthy value. Of these, the first is the least good. 

 The number of bands observed was only 15}, and the corrections for the "starting" 

 effect were of the order of -fo of this number. Any error in the estimate of this 

 correction, therefore, had a large proportional effect on the result. 



In the second experiment the " starting " effect was difficult to judge accurately. 

 I give the figures based on the estimate least favourable to concordant results. 



* TABLE IX. Refractive Index of Sulphur. 



