594 



POPULAR SCIENCE MONTHLY. 



The residues obtained from the evaporation of about thirty liters 

 of liquid air, after being freed from oxygen and nitrogen, were liquefied 

 by help of liquid air, and fractionated from each other. The separa- 

 tion offered no special difficulty, but was long and tedious. It soon 

 appeared that when most of the argon had been removed, the residue 

 solidified when cooled ; but while it was possible to remove the krypton 

 by pumping, for it goes into gas slowly even at the low temperature 

 of liquid air, very little xenon accompanied it ; for at that temperature, 

 xenon is hardly at all volatile. 



Having finally separated the gases, their densities and other prop- 

 erties were carefully determined; and it was also proved that they are 

 like argon and helium, in as much as their molecules consist of single 

 atoms. Neon, as was expected, turned out to be the missing link be- 

 tween helium and argon; the atomic weight of krypton was found to 

 be 81.6, and that of xenon, 128. The volumes occupied by equal num- 

 bers of molecules of the liquefied gases were determined; and also the 

 boiling-points and melting-points of argon, krypton and xenon. These 

 figures are shown in the following table: 



Helium. Neon. Argon. Krypton. Xenon. 



Density of gas 1.98 9.96 19.96 40.78 64.0 



Atomic weight 3.96 19.92 39.92 81.56 128.0 



Density of liquid 0.3 (?) 1.0(?) 1.212 2.155 3.52 



Boiling-points — — — 186.1°C. — 151.7°C. — 109.1 °C. 



Melting-points — — — 187.9°C. — 169.°C. — 140.°C. 



Critical temperatures.. — — — 117.4°C. — 62.5°C. + 14.75°C. 



Critical pressures — —(Metres.) 40.20 41.24 43.50 



Refractivity of gas 0.124 0.235 0.968 1.450 2.368 



In every case there is seen what is termed periodicity; that is, a 

 gradual alteration with rise of atomic weight, of the densities of the 

 liquids, of the melting-points, of the boiling-points, and of the retard- 

 ation of light when passed through the gas. 



Let us consider, in conclusion, the position of these elements in the 

 periodic table; and it will be sufficient to confine our attention to the 

 groups of elements which form the neighboring columns. The atomic 

 weights are given in round numbers. 



Hydrogen. Helium. Lithium. Beryllium. 



14 7 9 



Fluorine. Neon. Sodium. Magnesium. 



19 20 23 24 



Chlorine. Argon. Potassium. Calcium. 



35.5 40 39 40 



Bromine. Krypton. Rubidium. Strontium. 



80 82 85 87 



Iodine. Xenon. Caesium. Barium. 



127 128 133 137 



It is evident that these new elements fall into their natural places 

 between the strongly electro-negative elements of the fluorine group, and 



