10 



Professor Deicar 



[Jan. 20, 



Fig. 8. 



-' 



The mean of several observations gave the minimum deviation with 

 a prism of 59° 15' to be 15° 11' 30", and thence p = 1-2236. The 

 density of liquid oxygen at its boiling-point of —182° C. is 1*124, 



and this gives for the refraction-constant, ^— = 1*989, and for 



the refraction-equivalent 3*182. This corresponds closely with the 

 refraction-equivalent deduced by Landolt from the refractive indices 

 of a number of organic compounds. Also it differs little from the 

 refraction-equivalent for gaseous oxygen, which is 3*0316. This is 

 quite consistent with the supposition that the molecules of oxygen in 

 the liquid state are the same as in the gaseous. 



2 I 



If we take the formula , .. , ON , for the refraction-constant we 



find the value of it for liquid oxygen to be -1265, and the 

 corresponding refraction-equivalent 2*024. These are 

 exactly the means of the values found by Mascart and 

 Lorenz for gaseous oxygen. The inherent difficulties 

 of manipulation, and the fact that the sides of the 

 hollow prism invariably became coated with a solid 

 deposit, which obscured the image of the source of light, 

 have hitherto prevented our determining the refractive 

 indices for rays other than D. 



The optical projection of vacuum vessels having the 

 shape of a double test-tube are very suitable for lecture 

 illustration. As the critical point of oxygen is some 

 thirty degrees higher than nitrogen it is easier to liquefy, 

 and, consequently, becomes the most convenient substance 

 to use for the production of temperatures about — 200° C. 

 Liquid nitrogen, carbonic oxide, or air can conveniently 

 be made at the ordinary atmospheric pressure, provided 

 they are brought into a vessel cooled by liquid oxygen 

 boiling under the pressure of about half an inch of 

 mercury. 



A simple arrangement for this purpose is shown in 

 Fig. 8. The inner tube contains the liquid oxygen under 

 exhaustion, surrounded by a vacuum vessel, the interior space between 

 the inner tube and the vacuum vessel being connected with a 

 receiver containing the gas which is to be liquefied. If the object 

 is to collect liquid air, the inner air space is left quite open, no 

 precautions being needed to free the air from carbonic acid or 

 moisture, because under the conditions such substances are solids, 

 and only cause a slight opalescence in the liquid, which drops con- 

 tinuously from the end of the inner tube and accumulates in the 

 vacuum vessel. If the air supply is forced to bubble through a 

 little strong sulphuric acid, the rate of condensation and the rela- 

 tive volume of gas and liquid can be observed. Liquid air boils 

 at the temperature of - 190° C, giving off substantially pure nitrogen. 



