592 POPULAR SCIENCE MONTHLY. 



Again, stepping one pace down the scale, we have 19.5 as the difference 

 between chlorine and manganese; 20.3, between sulphur and chro- 

 mium; 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 ad- 

 dress: "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 pro- 

 cesses 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 pre- 

 pared 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 



