452 nREPORT—1883. 
ledged to be a fact by everyone more or less—that bodies emit, or cease to emit, 
different rays as their temperature increases, and notably when they pass from the 
liquid to the gaseous condition. And again, if the composition of the two layers of 
atmosphere be different, we have lately learnt how profoundly the admixture of a 
foreign substance will sometimes modify a luminous spectrum. 
2. Peculiarities of Atomic Weights—At the meeting of this Association at 
Ipswich, in 1851, M. Dumas showed that in several cases analogous elements form 
groups of three, the middle one of which has an atomic weight intermediate between 
those of the first and third, and that many of its physical and chemical properties 
are intermediate also. During the discussion upon his paper, and subsequently,} 
attention was drawn to the fact that this is not confined to groups of three, but 
that there exist many series of analogous elements having atomic weights which 
differ by certain increments, and that these increments are in most cases multiples 
of 8. Thus we have lithium, 7; sodium, 23, 7.c. 7 + 16; potassium, 39, ze. 7 + (16 x 2); 
and the more recently discovered rubidium, 85, 7.e. 7 + (16 x 5) nearly ; and czesium, 
133, t.e.7+(16 x8) nearly. This is closely analogous to what we find in organic 
chemistry, where there are series of analogous bodies playing the part of metals, 
such as hydrogen, methyl, ethyl, &c., differing by an increment which has the 
atomic weight 14, and which we know to be CH,. Again, there are elements with 
atomic weights nearly the same or nearly multiples of one another, instances of 
which are to be found in the great platinum group and the great cerium group.* 
This suggests the analogy of isomeric and polymeric bodies. There is also this re- 
markable circumstance : the various members of such a group as either of those just 
mentioned are found together at certain spots on the surface of the globe, and 
scarcely anywhere else. The chemist may be reminded of how in the dry distilla- 
tion of some organic body he has obtained a mixture of polymerised hydrocarbons, 
and may perhaps be excused if he speculates whether in the process of formation of 
the platinum or the cerium group, however and whenever it took place, the different 
elements had been made from one another and imperfectly polymerised. 
But this is not the largest generalisation in regard to the peculiarities of these 
atomic weights. Newlands showed that, by arranging the numbers in their order, 
the octaves presented remarkable similarities, and, on the same principle, Mendelejeft 
constructed his well-known table. I may remind you that in this table the atomic 
weights are arranged in horizontal and vertical series, those in the vertical series dif- 
fering from one another, asa rule, by the before-mentioned multiples of 8—namely 16, 
16,24, 24, 24, 24,32, 32—the elements being generally analogous in their atomicity 
and in other chemical characters. Attached to the elements are figures, represent- 
ing various physical properties, and these in the horizontal series appear as periodic 
functions of the atomic weights. The table is incomplete, especially in its lower 
portions, but, with allits imperfections and irregularities, there can be no doubt that 
it expresses a creat truth of nature. Now, if we were to interpolate the compound 
bodies which act like elements—methyl, 15; ammonium, 18; cyanogen, 26—into 
Mendelejeft’s table, they would be utterly out of place, and would upset the order 
both of chemical analogy and of the periodicity of the physical properties. 
3. Specific Refraction.—The specific refraction has been determined for a large 
majority of the elements, and is a very fundamental property, which belongs to 
them apparently in all their combinations, so long at least as the atomicity® is 
unchanged. If the figures representing this property be inserted into Mendelejeff’s 
table, we find that in the vertical columns the figures almost invariably decrease 
as the atomic weights increase. If, however, we look along the horizontal columns, 
or better still if we plot the figures in the table by which Lothar Meyer has shown 
graphically that the molecular volume is a periodic function of the atomic weights, 
1 Phil. Mag., May 1853. 
? Another curious instance is the occurrence of nickel and cobalt in all meteoric 
irons, with occasionally chromium or manganese, the atomic weights and other 
properties of which are very similar. 
’ This exception includes not merely such changes as that from a ferrous to a 
ferric salt, but the different ways in which the carbon is combined in such bodies as 
ethene, benzene, and pyrene. 
