494 
ready been claimed for one of the known 
elements is difficult, and the difficulty is 
enhanced when it is at the same time re- 
quired to select a name which shall be de- 
scriptive of the properties (or want of prop- 
erties) of the element. 
It is now my task to bring before you the 
evidence for the existence of this undiscoy- 
ered element. 
It was noticed by Doébereiner, as long ago 
as 1817, that certain elements could be ar- 
ranged 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 stron- 
tium, 42.5, is 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 Dober- 
einer, and such groups became known as 
‘ Dobereiner’s triads.’ 
Another method of classifying the ele- 
ments, also depending on their atomic 
weights, was suggested by Pettenkofer, and 
afterwards elaborated by Kremers, Glad- 
stone and Cooke. It consisted in seeking 
for some expression which would represent 
the differences between the atomic weights 
of certain allied elements. Thus, the differ- 
ence 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 conspicu- 
SCIENCE. 
[N.S. Von. VI. No. 144, 
ous; Dumas, in 1857, contrived a some- 
what 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 New- 
lands, 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, 
isa kind of repetition of the first, like the 
eighth note of an octavo in music.’ To this 
regularity he gave the name ‘The Law of 
Octaves.’ 
The development of this idea, as all chem- 
ists know, was due to the late Professor 
Lothar Meyer, of Tibingen, and to Pro- 
fessor Mendeléeff, 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 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 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, according to Lothar Meyer 
aud Mendeléeff, that the elements grouped 
down each of the vertical lines form a 
natural class ; they possess similar proper- 
ties, 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 consist- 
ing of three elements with approximately 
equal atomic weights. The elements in ques- 
tion are iron, cobalt and nickel; palladium, 
rhodium and ruthenium; and platinum, 
