340 Dr. L. Bleekrode. On the Determination of the 



II. Description of the Apparatus. 



Several methods are now at command in physics by which the 

 index of refraction may be determined even to the sixth decimal, yet 

 in this peculiar case, from the very conditions in which the liquids 

 were to be examined, it was not safe to employ prisms having glass 

 walls cemented on, as cements are often soluble in the liquefied 

 gases ; besides rather great dimensions would have been required, 

 rendering the effect of pressure the more difficult to resist. 



As in my former investigation, I chose again in most cases the 

 Faraday tubes, but I had them modified in the proper way, so as to 

 obtain the liquid inclosed between parallel glass sides, when I could 

 apply the microscope to make quantitative experiments. 



After many preliminary arrangements that were successively tried 

 I came to an equally effective and simple construction, that showed 

 itself most convenient and is represented in figs. 1 and 2, and I 

 suppose it may prove useful also in other researches with liquefied 

 gases. A section of the apparatus is given in fig. 1 ; it con- 

 sists of a short glass cylinder a (length 15 millims., inner diameter 

 3 millims., outer diameter 11 millims.), communicating on both sides 



