of Refraction in Selenium and Bromine. 53 



the absorption is confined to the violet end. If the film be 

 placed boiling under the spectro-microscope, the spectrum is 

 seen to bo generally darkened till near the red end, and the 

 violet quite cut off. As the film cools, the resulting dark 

 cloud seems to recede towards the violet. If the film be very 

 thin it appears colourless to the eye, and under the spectro- 

 scope the violet is visible. It remains colourless after the 

 sulphur has crystallized, appearing nearly white to the eye. 

 Selenium also darkens rapidly by heat and then transmits a 

 greyish light, which is probably due to its breaking up into 

 crystals. It has, how^ever, the property of becoming more 

 transparent as it cools. Thus the optical properties of these 

 two substances are as remarkably similar as are their che^ 

 mical relations. 



It will be observed that the quantity -— 7— ? though by no 



means the same in both selenium and sulphur, are not far 

 apart. There is, however, another relation which exists ; it 

 is expressed by the formula, that in selenium, 



i/se-l=(vs-l) ^; 



where vs is limit of refraction of sulphur, E its equivalent, and 

 d its density. That is, that the refraction-equivalent of sele- 

 nium can be found by multiplying the refraction-equivalent 

 of sulphur by the equivalent of selenium divided by the den- 

 sity of that element. This is worthy of remark, as it appears 

 that the same relation within five or six units in the second 

 place of decimals between isobutyl iodide as compared with 

 isobutyl chloride, and orthobromotoluine compared with chloro- 

 toluine and benzyl chloride. With regard to benzyl chloride, 

 I may mention that calculation for the upper limit has revealed 

 an error due to extracting the wrong logarithm of sin d^. The 

 correct result given below agrees better with the rule that 

 J/ — 1 



— T— is constant in isomeric bodies. As in the case of two 

 d 



isomeric bodies, the equivalent is the same in both, it will be 

 seen that this result is in agreement with the relation be- 

 tween selenium and sulphur stated above. 



It is worthy of note that chlorine and bromine resemble 

 sulphur and selenium in tint of transmitted light. The index 

 of refraction for the line A in bromine is /xa = 1*6260. It 

 occurred to me that it would be possible to obtain a probable 

 idea of the spectrum of this element by assuming an upper 

 limit of refraction. Taking this limit as D and F, to which 

 I subsequently added G, I found that F gave ^a=39° 44', 



