Again, stepping one pace down the scale, we have 19.5 as the difference between chlorine and manganese; 20.3, between sulphur and chromium; 19.8, between silicon and titanium, etc. The total difference between manganese and fluorine is 36; between chromium and oxygen, 36.3; between vanadium and nitrogen, 37.4, and between titanium and carbon, 36.1. This is approximately the difference between the atomic weights of helium and argon, 36. I quote now from that address: "There should, therefore, be an undiscovered element between helium and argon, with an atomic weight 16 units higher than that of helium, and 20 units lower than that of argon, namely 20. And if this unknown element, like helium and argon, should prove to consist of monatomic molecules, then its density should be half its atomic weight, 10. And pushing the analogy still further, it is to be expected that this element should be as indifferent to union with other elements as the two allied elements."
Those who care to read the story of the search for this undiscovered element may find it in the address. Minerals from all parts of the globe, mineral waters from Britain, France and Iceland, meteorites from interstellar space; all these were investigated without result. Helium from various minerals was separated by long and tedious processes of diffusion into a possibly lighter portion, diffusing more rapidly, and a possibly heavier portion, diffusing more slowly; but with no positive result. The systematic diffusion of argon, however, gave a faint indication of where to seek for the missing element, for the density of the more rapidly diffusing portion was 19.93, while that of the portion which diffused more slowly was 20.01.
The invention by Dr. Hampson of an apparatus by means of which it is possible to obtain liquid air at small expense and with little trouble placed a new instrument in our hands; and Dr. Travers and I prepared 15 liters of argon from the atmosphere, with the purpose of distilling it fractionally, after liquefaction; for we knew, from the researches of Professor Olszewski of Cracow, who has done so much to determine the properties of liquefied gases, that argon could be liquefied easily by compressing it into a vessel cooled by help of liquid air. And, moreover, we were in hope that by fractionating the air itself, gases of even higher atomic weight than argon might possibly be obtained. Both expectations were realized; on distilling liquid argon, the first portions of gas to boil off were found to be lighter than argon; and on allowing liquid air to boil slowly away, heavier gases came off at the last. It was easy to recognize these gases by help of the spectroscope; for the light gas, to which we gave the name, neon, or 'the new one,' when electrically excited emits a brilliant flame-colored light; and one of the heavy gases, which we called krypton, or 'the hidden one,' is characterized by two brilliant lines, one in the yellow
