1:885.1 Recent Developments of Chemical Science, 19 
It must be observed that the above law refers only to 
cases in which A, B, C, &c., are elements belonging to the 
same sub-group, but not to cases where they belong to 
different sub-groups. Thus oxides cannot be direCtly com- 
pared. with the corresponding sulphides, selenides, and 
tellurides. Nor can fluorides be compared with the corre- 
sponding chlorides, bromides, and iodides, for fluorine and 
oxygen belong to even series, whilst chlorine, bromine, 
iodine, and sulphur, selenium, and tellurium pertain to odd 
ones. 
It must likewise be remembered that the above rule 
applies strictly only to solid compounds, and that in the 
solid state the colour can within certain limits change with 
the condition of aggregation. 
In addition to 426 cases in which the law has been veri- 
fied, Prof. Carnelley meets with fourteen exceptions. Four 
of these, DiCl 3 , V 2 0 5 , Cr 0 3 , and CaO, do not seem to admit 
of any satisfactory explanation. He considers that the 
colour of a compound will ultimately prove to be a periodic 
function of its atomic weight. This may best be seen on 
considering the normal iodides. If we construct a curve in 
which the ordinates represent the atomic weights of the 
positive elements, and the abscissae form a scale rising from 
black, through brown, red, orange, yellow, green, &c., to 
white, the curve thus obtained resembles in figure Lothar 
Meyer’s well-known elemental curve. 
Could Prout’s hypothesis — i.e., that the atomic weights of 
all the elements are multiples by whole numbers of that of 
hydrogen ( — 1) — be demonstrated, it would have contributed 
no little to prove that these bodies are genetically connected. 
But the most careful re-determinations of the atomic 
weights lead us in the very opposite direction. Instead of 
the exceptions to the supposed law disappearing, on more 
exaCt scrutiny they persist, or are even increased. 
One indirect consideration in favour of a common evolu- 
tionary origin of the elements has, perhaps, scarcely received 
the attention which it deserves, — i. e., the occurrence in in- 
timate admixture of bodies having very similar properties, 
and sometimes even closely approximating atomic weights. 
Such groups are, e.g., cobalt and nickel, the three halogens, 
and especially the rare earths found together in samarskite, 
&c. It is difficult to frame a hypothesis which will account 
for the co-existence of these various groups of bodies, save 
the one that they have been evolved out of some common 
material. 
But this, and all other indirect evidence pointing in the 
