Index of Refraction of Liquefied Gases. 343 



lower part of the tube, in the upper one some compounds fit to absorb 

 any moisture present in the evolved gas, and both ends sealed with 

 the blowpipe. Afterwards the substances were put in contact by 

 gently shaking the tube, and the operation conducted so as to have 

 the liquefied gas distilling over into the vessel, directed with its axis 

 in a horizontal position. Enough liquid was always obtained to fill 

 it completely and generally also the connected capillary tubes on both 

 sides, and no gas or air remained ; the small dimensions of this part 

 of the apparatus caused some drops of liquid to be sufficient. The 

 liquefying operation conducted with the Cailletet pump proceeded in 

 the usual way ; the capillary part 6 (fig. 1) of the vessel was fixed with 

 the blowpipe' on the ordinary gas tube of this apparatus, the other 

 capillary part c was closed after this tube was filled with the purified 

 gas. 



If the gas could be put readily into the liquid condition with the 

 aid of low temperatures, the cylindrical vessel was put on a doubly 

 bent glass tube, and this surrounded by a freezing mixture ; this was 

 applied in the case of sulphurous acid, some amine compounds, &c. 



Lastly, I will observe, that after terminating a series of experiments 

 with some gas, the whole apparatus could be liberated from its 

 contents by cutting off a very small part of the lower end of the tube, 

 and all remaining substances were removed by the pressure of the 

 gas ; thus I could afterwards examine the glass disks and convince 

 myself whether the liquefied gases, being perhaps not pure, had left 

 behind some moisture or other deposits. 



III. The Method of Observation. 



The reason why I had to provide for getting the liquefied gas 

 inclosed between parallel glass plates was to have an opportunity to 

 apply the microscope, which perhaps may be the only practical effec- 

 tive method under these circumstances. 



The use of this instrument in determining the index of refraction 

 was first proposed by the Duke de Chaulnes in 1767, but the exacti- 

 tude of the results being limited to the third decimal (from the 

 principle on which the method is based) it is inferior to many other 

 methods, and was, of course, neglected in physics.* The mathe- 

 matical expression for the index that has to be used in this case 

 follows from very simple considerations. 



* In 1849 Bertin published some modifications of this method, without improving, 

 however, its exactitude, as appears from the values communicated for the index of 

 glass (1-497 and T703), the goniometer giving T507 and T696. 



More recently, in 1876, Eoyston Pigott, when describing his refracto meter, com- 

 mented on adapting his instrument to the microscopical method, and in 1878 

 Mr. Sorby, of Sheffield, made several valuable and interesting communications on 

 the refractive indices of minerals, which were determined with the microscope, and 

 he also pointed out the practical use in the case of small quantities of liquids. 



