62 
PROFESSOR W. N. HARTLEY ON SPECTRUM PHOTOGRAPHY, ETC. 
steel and dry iridium electrodes yielded spectra "with the calcium lines. In the case 
of the steel, either minute traces of slag are diffused through the metal or traces of 
calcium occur as one of its constituents. The iridium points were new, and it is just 
conceivable that traces of lime from the crucible in which the metal was fused were 
adhering to its outer surface. The specimens of graphite contained magnesium and 
iron, but no calcium. This conclusion was the result of spectrum observations, and 
likewise of a careful analysis of the ash made by the usual methods. Many of the 
photographs of solutions of metallic chlorides, as for instance the chlorides of copper 
and manganese, show no trace of the calcium lines. I have now no doubt that when 
the calcium salt is not derived from the action of acids on glass vessels, the spectra 
are contaminated by dust floating in the air, and that the calcium lines are to the 
ultra-violet region in this respect what the sodium lines are to the visible spectrum. 
The alkali-metals yield but feeble ultra-violet spectra, otherwise the sodium lines 
would likewise be ever present. 
The evidence that the calcium lines are due to dust is the following : two metallic 
electrodes, alloys of copper and silver, were filed up so as to present bright points ; 
they were each opposed to a graphite electrode, and the spark -was made to pass from 
the metal to the carbon ; in the one case the calcium lines were either very faint or 
invisible, but in the second they were very strong. The interval of time bekween the 
taking of the photographs was not more than half a minute. The calcium lines were 
strongest at the metallic or negative pole. 
The following facts have been established by the foregoing observations :— 
When carbon or metallic electrodes are moistened the short lines are lengthened. 
With very few exceptions the non-metallic constituents of a salt do not affect the 
spark spectra of solutions. 
Insoluble and non-volatile compounds do not yield spark spectra. 
The solution of a metallic chloride yields spectral lines identical in number and 
position with the principal lines of the metal itself. 
Short lines become long lines, but otherwise their character is identical, whether the 
spectra are produced by metallic electrodes or solutions. 
The effect of diluting solutions of metallic salts is first to weaken and attenuate the 
metallic lines, then with a more extensive dilution to shorten them, the length of the 
longest and strongest lines generally decreasing until they finally disappear. 
Accidental differences in the passage of the spark or in the time of exposure of the 
photographic plate, when the normal period is from half a minute to five minutes, do 
not cause sensible variations in spectra obtained from the same substance. 
