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 separation 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 properties 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 between helium and argon; the atomic weight of krypton was found to be 81.6, and that of xenon, 128. The volumes occupied by equal numbers 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 retardation 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. |
| 1 | 4 | 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. | Cæsium. | 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
