SPECTEA OF ALLOTS. 
883 
of copper and 38 of zinc, gave a spectrum in which the lines due to copper predominated 
considerably. In an alloy of about 2 parts of zinc to 1 of cadmium the zinc-spectrum 
was the most strongly marked. 
In another experiment an alloy of 990 parts of fine gold and 10 of fine silver was pre- 
pared ; on taking the spectrum obtained by an exposure of 10 minutes, a distinct but 
feeble impression of the more refrangible lines due to silver was procured (see fig. 8). 
A contamination of gold with silver to an extent not exceeding 1 per cent, could there- 
fore be recognized by this means ; but prolonged exposure was necessary in order to 
develope the lines due to silver. An analogous result was obtained when the spectrum 
of plumbago was taken. In this case, in addition to the atmospheric lines, the spectrum 
of iron was distinctly impressed : the total amount of metallic iron in the plumbago was 
3’94 per cent. Graphite deposited in the gas-retorts, which contained 0'23 per cent, of 
iron, gave very feeble indications of iron. On the other hand, no indication of iron was 
observable in the spectrum of brass which contained 0’23 per cent, of iron, nor was lead 
indicated in brass which contained 0'7 per cent, of this metal. 
58. All the foregoing spectra were obtained either from the metals in their uncombined 
form, or else from their alloys. The electric spectra of a few other metals which admit 
of being submitted to experiment in their isolated form, still remain to be added. A 
considerable proportion of the metallic elements, however, are not included in the fore- 
going list. These it is almost impossible to examme, except in the form of some of 
their saline or other compounds. As, however, this portion of the inquiry is attended 
with some peculiar difficulties, I shall defer what I have to add upon this subdivision 
of the subject to a future occasion. 
§ 4. PHOTO GEAPHIC EFFECTS OF ELECTEIC SPECTEA OF DIFFEEENT METALS PEO- 
DHCED BY TEANSMITTING THE SPAEKS THEOHGH GASES OTHEE THAN 
ATMOSPHEEIC AIE. 
59. In making experiments upon the infiuence of various gases upon the spectra of the 
electric spark, the arrangement of the apparatus was modified in the following manner : 
— The position of the slit, prism, lens, and camera was the same as in the preceding ex- 
periments (6); but the metallic electrodes were enclosed in a stout glass tube, shown at 
half its real size in Plate XT, , fig. 49. a is the tube itself, h a hole drilled through the 
side of the tube, which upon this side is ground flat in order that it may be closed air- 
tight by the thin plate of polished quartz c. This plate is kept in its place by means 
of an elastic band, d, d are brass forceps screwed into the brass plugs e, e, for holding 
the electrodes. The ends of the tube are closed by the brass plugs e, e, which are 
ground to fit the ends of the tube, and are pierced by small brass tubes for the con- 
veyance of the gas. An elastic band, passing from one end of the glass tube to the 
other, keeps the brass plugs in their place. The tube is then connected with a gas-holder 
