362 On the Index of Refraction of Liquefied Gases. 



VI. Appendix. 



Determination of the Index of Refraction of Liquid Bromine and some 

 Organo-Metallic Compounds. 



I add to this paper the results of experiments with some substances 

 that, though they are liquid at ordinary temperature, yet present some 

 difficulties on being examined by the common methods ; their index 

 of refraction has also till now not been observed directly. 



The practical efficiency of the microscopical method is well illus- 

 trated in the case of liquid bromine, which is very opaque even in thin 

 layers. I had a glass cell constructed with parallel sides in a simple 

 way, by cementing together two plane glass disks, one of them 

 consisting of 'a thin glass cover (as used in microscopical prepara- 

 tions), with a hole in the centre ; this forming the proper vessel when 

 it was finally covered with a glass disk, a single drop of liquid 

 bromine being previously put between. Supplying a 140 magnifying 

 power, enough light (daylight) was transmitted through the layer to 

 permit the focussing on scales of Lepidoptera wings. This layer 

 measured 0'33 millim., the apparent displacement 0*12 millim. (as 

 a mean result of repeated observations) ; from this the index of 

 refraction is calculated as 1 '5 71 at a temperature of 13. 



I examined also some organo-metallic compounds, especially those 

 which are spontaneously ignited on contact with air. I employed 

 doubly bent glass tubes, ending in the parallel-sided vessel and filled 

 previously with carbonic acid gas. Then I distilled into them from 

 another tube the liquid compound prepared in the usual way, a very 

 small quantity being of course sufficient. I add the following results, 

 referring to the natrium light : 



n. Temp. 



Zinc ethyl 1 '485 12 '5 



Zinc methyl 1 '474 14 



Aluminium ethyl ... 1 '480 6 '5 



Aluminium methyl .. 1*432 12 



If these values are compared to those of other metallic compounds, 

 as for instance the tetrachloride of tin (SnCl 4 ), its index amounting 

 to 1'5225* (for the ft line of the spectrum of hydrogen), we may 

 remark that it is not necessary for a substance to be very combustible 

 in order to have a very high index of refraction, an opinion that some- 

 times has been pronounced. 



Haagen, in " Pogg. Ann." (1867), Bd. 151, S. 122. I may also add analogous 

 compounds, as mercury ethyl (l - 5397, D line) and mercury methyl (1*5319, 

 1) line). 



