August 19, 1897] 



NATURE 



379 



The subject of my remarks to-day is a new gas. I shall 

 describe to you later its curious properties ; but it would be 

 unfair not to put you at once in possession of the knowledge of 

 its most remarkable property — it has not yet been discovered. As 

 it is still unborn, it has not yet been named. The naming of a 

 new element is no easy matter. For there are only twenty-six 

 letters in our alphabet, and there are already over seventy 

 elements. To select a name expressible by a symbol which has 

 not already been claimed for one of the known elements is diffi- 

 cult, and the difficulty is enhanced when it is at the same time 

 required to select a name which shall be descriptive of the pro- 

 perties (or want of properties) of the element. 



It is now my task to bring before you the evidence for the 

 existence of this undiscovered element. 



It was noticed by Dobereiner, as long ago as 1817, that 

 certain elements could be arranged in groups of three. The 

 choice of the elements selected to form these triads was made 

 on account of their analogous properties, and on the sequence of 

 their atomic weights, which had at that time only recently been 

 discovered. Thus calcium, strontium, and barium formed such 

 a group ; their oxides, lime, strontia, and baryta are all easily 

 slaked, combining with water to form soluble lime-water, 

 strontia-water, and baryta-water. Their sulphates are all 

 sparingly soluble, and resemblance had been noticed between 

 their respective chlorides and between their nitrates. Regularity 

 was also displayed by their atomic weights. The numbers then 

 accepted were 20, 42*5, and 65 ; and the atomic weight of 

 strontium, 42*5, is the arithmetical mean of those of the other 

 two elements, for (65 -f-2o)/'2 = 42-5. The existence of other 

 similar groups of three was pointed out by Dobereiner, and such 

 groups became known as " Dobereiner's triads." 



Another method of classifying the elements, also depending 

 on their atomic weights, was suggested by Pettenkofer, and 

 afterwards elaborated by Kremers, Gladstone, and Cooke. It 

 consisted in seeking for some expression which would represent 

 the differences between the atomic weights of certain allied 

 elements. Thus, the difference between the atomic weight of 

 lithium, 7, and sodium, 23, is 16 ; and between that of sodium 

 and of potassium, 39, is also 16, The regularity is not always 

 so conspicuous ; Dumas, in 1857, contrived a somewhat com- 

 plicated expression which, to some extent, exhibited regularity 

 in the atomic weights of fluorine, chlorine, bromine, and iodine ; 

 and also of nitrogen, phosphorus, arsenic, antimony and bismuth. 

 The upshot of these efforts to discover regularity was that, in 

 1864, Mr. John Newlands, having arranged the elements in 

 eight groups, found that when placed in the order of their 

 atomic weights, " the eighth element, starting from a given one, 

 is a kind of repetition of the first, like the eighth note of an 

 octave in music." To this regularity he gave the name "The 

 Law of Octaves." 



The development of this idea, as all chemists know, was due 

 to the late Prof. Lothar Meyer, of Tubingen, and to Prof. Men- 

 deleeff, of St. Petersburg. It is generally known as the 

 " Periodic Law." One of the simplest methods of showing this 

 arrangement is by means of a cylinder divided into eight seg- 

 ments by lines drawn parallel to its axis ; a spiral line is then 

 traced round the cylinder, which will, of course, be cut by these 

 lines eight times at each revolution. Holding the cylinder ver- 

 tically, the name and atomic weight of an element is written at 

 each intersection of the spiral with a vertical line, following the 

 numerical order of the atomic weights. It will be found, accord- 

 ing to Lothar Meyer and Mendeleeff, that the elements grouped 

 down each of the vertical lines form a natural class ; they pos- 

 sess similar properties, form similar compounds, and exhibit a 

 graded relationship between their densities, melting-points, and 

 many of their other properties. One of these vertical columns, 

 however, differs from the others, inasmuch as on it there are 

 three groups, each consisting of three elements with approxi- 

 mately equal atomic weights. The elements in question are 

 iron, cobalt, and nickel ; palladium, rhodium, and ruthenium ; 

 and platinum, iridium, and osmium. There is apparently room 

 for a fourth group of three elements in this column, and it may 

 be a fifth. And the discovery of such a group is not unlikely, 

 for when this table was first drawn up Prof. Mendeleeff drew 

 attention to certain gaps, which have since lieen filled up by the 

 discovery of gallium, germanium, and others. 



The discovery of argon at once raised the curiosity of Lord 

 Rayleigh and myself as to its position in this table. With a 

 density of nearly 20, if a diatomic gas, like oxygen and nitrogen, 

 it would follow fluorine in the periodic table ; and our first idea 



was that argon was probably a mixture of three gases, all of 

 which possessed nearly the same atomic weights, like iron, 

 cobalt, and nickel. Indeed, their names were suggested, on 

 this supposition, with patriotic bias, as Anglium, Scotium, and 

 Hibernium ! But when the ratio of its specific heats had, at 

 least in our opinion, unmistakably shown that it was molecularly 

 monatomic, and not diatomic, as at first conjectured, it was 

 necessary to believe that its atomic weight was 40, and not 20, 

 and that it followed chlorine in the atomic table, and not 

 fluorine. But here arises a difficulty. The atomic weight of 

 chlorine is 35 5, and that of potassium, the next element in order 

 in the table, is 39"i ; and that of argon, 40, follows, and does 

 not precede, that of potassium, as it might be expected to do. 

 It still remains possible that argon, instead of consisting wholly 

 of monatomic molecules, may contain a small percentage of 

 diatomic molecules ; but the evidence in favour of this supposi- 

 tion is, in my opinion, far from strong. Another possibility is 

 that argon, as at first conjectured, may consist of a mixture of 

 more than one element ; but, unless the atomic weight of one of 

 the elements in the supposed mixture is very high, say 82, the 

 case is not bettered, for one of the elements in the supposed trio 

 would still have a higher atomic weight than potassium. And 

 very careful experiments, carried out by Dr. Norman Collie and 

 myself, on the fractional diffusion of argon, have disproved the 

 existence of any such element with high atomic weight in argon, 

 and, indeed, have practically demonstrated that argon is a 

 simple substance, and not a mixture. 



The discovery of helium has thrown a new light on this sub- 

 ject. Helium, it will be remembered, is evolved on heating 

 certain minerals, notably those containing uranium ; although it 

 appears to be contained in others in which uranium is not 

 present, except in traces. Among these minerals are cleveite, 

 monazite, fergusonite, and a host of similar complex mixtures, 

 all containing rare elements, such as niobium, tantalum, yttrium, 

 cerium, &c. The spectrum of helium is characterised by a re- 

 markably brilliant yellow line, which had been observed as long 

 ago as 1868 by Profs. Frankland and Lockyer in the spectrum of 

 the sun's chromosphere, and named *' helium " at that early 

 date. 



The density of helium proved to be very close to 2'0, and, 

 like argon, the ratio of its specific heat showed that it, too, was 

 a monatomic gas. Its atomic weight therefore is identical with 

 its molecular weight, viz. 4*0, and its place in the periodic 

 table is between hydrogen and lithium, the atomic weight of 

 which is 7 o. 



The difference between the atomic weights of helium and 

 argon is thus 36, or 40 - 4. Now there are several cases of such 

 a difference. For instance, in the group the first member of 

 which is fluorine we have — 



Fluorine 



Chlorine.. 



Manganese 



In the oxygen group — 



Oxygen 



Sulphur 



Chromium 



55 



[9-5 



16 



16 



32 20.3 



52-3 ^ 



In the nitrogen group — 

 Nitrogen 

 Phosphorus 

 Vanadium 



And in the carbon group- 

 Carbon ... 

 Silicon ... 

 Titanium 



14 

 31 

 51-4 



17 

 20 '4 



t^i 



NO. 1 45 I, VOL. 56] 



These instances suffice to show that approximately the differ- 

 ences are 16 and 20 between consecutive members of the corre- 

 sponding groups of elements. The total differences between the 

 extreme members of the short series mentioned are- 



Manganese - Fluorine 

 Chromium - Oxygen... 

 Vanadium - Nitrogen 

 Titanium - Carbon ... 



36 

 363 

 37*4 

 36-1 



This is approximately the difference between the atomic weights 

 of helium and argon, 36. 



