886 
PEOFBSSOR W. A. MILLEE ON THE PHOTOGEAPHIC 
of gas traversed by the rays before they entered the air amounted to about half an inch 
in thickness. 
67. Sulphuretted Hydrogen. — This gas also offers considerable resistance to the passage 
of the electric spark. It is decomposed by the spark with deposition of sulphur. When 
gold electrodes are used, it furnishes lines resembling those of the same metal in air. 
With silver electrodes the gas was decomposed very rapidly, and no lines were produced 
beyond 113-5, the absorbent action of the gas being strongly manifested. 
68. Ammonia. — Sparks pass in this gas as freely as in air ; the spectrum of each metal 
is the same as in nitrogen ; no new lines are visible in the photograph. Most of the 
atmospheric lines are distinct. 
69. Protoxide of Nitrogen. — The electric sparks pass in this gas with much greater 
difficulty than in air. The spectrum appears to be the same as that produced in air, 
and no new lines are apparent in the photograph. 
70. Nitrogen. — The spectrum of this gas, when gold or platinum electrodes are used, 
commences with a pale continuous spectrum, which slowly diminishes, in intensity ; this 
continuous spectrum appears to increase in intensity with the volatility of the metal, 
being well-marked in the case of magnesium, sodium, and potassium : at about 151 it 
terminates abruptly. The spectrum of nitrogen is crossed between 96'5 and 100 by 
two strong double lines; it shows an indistinct line at 108-5, a strong one at 110-5, 
three feeble lines at 113-5, 118, and 122, a faint band at 138, and another at 150, 
Fig. 33 shows the lines obtained from platinum points in nitrogen. 
71. Oxygen. — This gas was obtained from black oxide of manganese heated with 
sulphuric acid. It gave, after purification by passing through a solution of caustic soda, 
lines identical with many of those obtained in atmospheric air. When the gas con- 
tained traces of carbonic acid, the lines due to the compounds of carbon were distinctly 
visible in the impressed spectrum. With platinum electrodes and with pure oxygen a 
feeble, nearly continuous spectrum extends to about 122-5 ; it also contains numerous lines 
extending as far as 142-5 ; beyond that, the impression is more feeble, terminating at about 
156. The principal lines due to the gas are the following: — A broad line about 100, 
then two faint lines, beyond which, at 101-5, is a double line; a strong complex group at 
103-5 ; a feebler one at 105-5 ; one rather stronger at 107-5 ; a double group of consider- 
able strength at 112 ; another stronger at 114 ; between 116 and 119 is a group of six 
rather faint lines ; after this there are no prominent lines until 138-2 and 141-5 ; at 
153 is a strong compound line, and beyond this only the dotted lines of platinum ai'e 
seen. (Fig. 32.) 
72. Chlorine and Hydrochloric Acid give spectra which can scarcely be distinguished 
one from the other. With platinum points these spectra terminate by an abrupt band 
at 156-5. Their most marked features are a strong compound band at 96-5, and one still 
more marked at 100 ; then two lines, of which the first at 103 is the stronger, followed 
by two others, of which the second at 108-5 is the stronger: several fainter lines follow 
these lines. A group of six between 126 and 133, the most marked of which is a broad 
