594 REPORT— 1897. 



the arithmetical mean of those of the other two elements, for (65 + 20)/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 Pottenkofer, 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, 2.3, 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 complicated 

 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 pl.iced 

 in the order of their atomic weights, ' the eighth element, starting from a given one, 

 is a kind of repetition of the tirst, 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 

 Professor Lothar Meyer, of Tiibingen. and to Professor MendeMeff', of St Peters- 

 burg. 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 segments 

 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 vertically, 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, according to Lothar Meyer and Men- 

 deliSeff, that the elements grouped down each of the vertical lines form a natural 

 class; they possess 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, diHers from the others, 

 inasmuch as on it there are three groups, each consisting of three elements with 

 approximately 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 Professor Mendelileft" drew attention to 

 certain gaps, which have since been filled up by the discovery of gallium, ger- 

 manium, 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 moleculavly 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' 1 ; 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 supposition 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 



