214 Proceedings of Royal Society of Edinburgh. [may 2, 
metals may not be of the same colour, but we find that, in passing 
to higher members of the group from the lower ones, a uniform 
change in colour is to be observed. This change is produced by a 
gradual shifting of the absorption towards the red end of the 
spectrum, the molecules vibrating more and more slowly with in- 
crease of atomic weight. This can be illustrated by the following 
diagram taken from his paper : — 
Metal. 
Cl 
I 
Metal. 
Cr0 4 
As0 4 
Na 
White 
White 
Mg 
Lemon yellow 
White 
Cu 
White 
Cream 
Zn 
Yellow 
White 
Ag 
White 
Light yellow 
Cd 
Orange yellow 
White 
Au 
Yellow white 
Golden yellow 
Hg 
Eed 
Yellow 
It is probable, then, that metals of the same group vibrate in a 
similar way. This vibration we know is complex, consisting of 
many wave-lengths of different pitch. When a metal vibrates, and 
one of its vibrations falls within the scale of the visible spectrum, 
we shall find the corresponding wave of another member of the 
same group in the neighbourhood. If a higher member of the 
group, it will absorb the light nearer the red end ; if a lower, 
nearer the blue end of the spectrum. Together with this alteration 
of pitch, we have corresponding alterations in the sensations pro- 
duced, whether they be of sight, taste, or smell. Whatever reason 
we have for associating quality of colour with the pitch of vibration, 
we shall likewise have for associating quality of smell and taste 
with the same physical cause.* 
Amongst organic substances many are so closely allied that they 
fall into distinct classes or groups. Thus we have the fatty acids, 
alcohols, &c. If these be arranged in homologous series, commencing 
with that which has the lowest, and passing to that which has the 
highest molecular weight, a uniform change will be observed in many 
physical properties on ascending the series. Thus the lower ones 
may be gaseous, the middle ones liquid, and the higher ones solid. 
* If a curve be constructed in which the ordinates represent the atomic 
weights of the positive elements, and the abscissae a chromatic scale arising 
from blue, green, &c., to black, we shall obtain a curve indicating that the 
colours of the compounds are a periodic function of the elements arranged in 
atomic series. This is well seen in the case of the normal iodides (Carnelley). 
