SPECTEA OF THE METALS. 
molybdenum, chromium, and manganese were reduced from pure oxides in crucibles lined 
with charcoal. The other metals were as they are furnished in commerce as pure. 
26. Each metal gives its own distinctive spectrum ; but it is remarkable that these 
differences are not obvious in the less refrangible end. The true metallic spectrum, when 
the sparks pass in air, is in fact combined with that due to atmospheric air, as has already 
been pointed out for the visible rays by Angstkom and by Alter. The photographic 
lines of the air-spectrum are most marked in the less refrangible portion, whilst the 
characteristic lines of the metals are particularly e\ident in the more refrangible parts. 
Hence the photographs which I formerly obtained by the use of a prism of bisulphide 
of carbon, which transmits rays of low refrangibility only, represent, as I then correctly 
pointed out, lines which are chiefly atmospheric ; and consequently they exhibit ap- 
pearances which are almost identical whatever be the metal employed. 
In describing the spectra of the different metals, I shall employ the same arbitrary 
scale that I have hitherto used in this paper. 
27. It wiU be observed that generally the lines as they advance towards the less re- 
frangible extremity become less intense in their central portion, until towards the 
extreme limit of the spectrum, the two marginal ends of the lines alone are visible, 
though these terminations are often rather intense. Indeed, throughout the whole 
length of the impressed photograph, the marginal extremities of the metallic lines leave 
a stronger image than their central portions, as though the incandescence of the volati- 
hzed portions of the electrodes, owing to their high radiating power, did not continue 
sufficiently intense during their transfer across the interval between the two electrodes, 
to enable them to produce a continuous line. Evidently the cause of this diminution 
of action operates more powerfully upon the more refrangible rays ; and a higher 
temperature, as the experiments of other observers have abundantly proved, is necessary 
to the production of radiations of these high degrees of refrangibility. 
Exceptions to this remark occur in the lines due to the atmosphere ; this is well seen 
in the strong line at IIO'S, which is in marked contrast to some of the metallic lines in 
its vicinity, particularly in the spectrum of silver (Plate XXXIX. flg. 9), where this 
nitrogen-line is included between two pairs of very intense lines due to the metal itself, 
and which are each interrupted in the middle. 
In order to abbreviate the description of the various spectra, I shall generally speak' 
of these interrupted lines as “ dots”', they, indeed, constitute the characteristic features 
of the different metallic spectra. These dots, if the image be exactly in focus, may 
usually be seen to consist of groups of very short lines closely aggregated. This is well 
shown in some parts of Plate XT; , fig. 39, which represents the spectrum of silver ; and 
it is less distinctly shown in the spectra of palladium (fig. 38), of copper (fig. 40), of 
antimony (fig. 41), and of cadmium (fig. 42). These spectra were taken with the screen, 
lens, and prism at a distance from the slit different from those with which the other 
impressions were procured ; some parts are consequently out of focus, but the details of 
other portions are shown more fully. 
MDCCCLXII. 6 D 
