Band and Line Spectra of the same Metallic Elements. 13] 
Though the constitution of these banded spectra is very different from that of 
the calcium group of metals, yet the connexion between the band and line spectra 
of each element is a very intimate one, inasmuch as the lines appear to be 
developed from the bands. This is more particularly observable in the case of 
the most volatile metal of the group, cadmium. No such connexion is to be 
traced in beryllium, aluminium, and lanthanum band spectra, partly because of 
the difficulty in reducing their oxides, and in part because of their high boiling 
points. 
The Tetrad Elements. 
The banded spectra of tin and lead which belong to this group are of quite a 
different constitution from those belonging to any of the foregoing elements. 
They are fluted or channelled spectra. 
There is a difference in behaviour in the two elements, probably due to a 
difference in their boiling points, that of tin being the higher. 
TIN. 
The banded spectrum of tin extended the whole length of the plate from 
\ 5000 to X 8250. The bands have sharp edges, but no lines are visible, and by 
reducing the quantity of substance in the flame the bands become feeble through- 
out, and are finally extinct when the quantity of substance is reduced to less than 
1 centigramme. 
LEAD. 
The fluted bands degraded towards the red extend the whole length of the 
spectrum, but are strongest between \ 4900 and » 4058. These bands are 
accompanied by three lines, \A 4058, 3682, and 3639, all strong. 
On reducing the quantity of substance in the flame, the lines survive after 
the bands are extinct. The ultimate line is at \ 4058, and is caused by 75 of a 
milligramme of lead. The last appearance of the bands is caused by 1 centi- 
gramme of the metal. 
On the Origin of different Bands. 
There are bands in the spectra of the elements which have their origin in 
three conditions of matter in the flame. First, there are those which are without 
any sharp outline, ill-defined, or diffuse. ‘The most pronounced are the bands in 
the flame spectrum of silver, in the blue region, and the bands in the spectra of 
the alkali metals, ‘They are photographed only when large volumes of vapour 
