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XIV. On the Quantitative Analysis of certain Alloys hy means of the Spectroscope. 
By J. Norman Lockyer, F.B.S . , and W. Chandler Eoberts, Chemist of the Mint. 
Received November 20, — Read November 27, 1873. 
In certain researches on Spectrum-Analysis, the results of which have already been 
communicated by one of us to the Eoyal Society, it has been shown that the spectroscope 
may be employed to distinguish minute differences in the composition of certain alloys. 
We have therefore instituted a series of experiments with the view to ascertain the 
degree of accuracy of which the method is capable, and now submit the results which 
we have already obtained. 
In the researches above referred to, an electric-spark-current was used to volatilize 
the alloy under examination, and the image of the spark was thrown by means of a lens 
on the slit of a spectroscope ; the phenomena observed indicated that a change in the 
percentage of any constituent of a metallic alloy ordinarily causes a change in the cha- 
racter of lines exhibited in the resulting spectrum, in length, brightness, or thickness ; 
and the percentages of the constituents, it was stated, might be determined by compa- 
rison with the spectra of pieces of metal of known composition. 
It may be remarked that changes are exhibited in the spectra in various ways, 
thus : — 
1. Changes in the lengths of lines of one constituent. 
2. Changes in the position of the point at which lines belonging to different metals 
in the alloy show equal brightness or thickness. 
3. Changes in the relative lengths of a pair of lines belonging severally to the con- 
stituents of the alloy. 
Any of the above changes may give the requisite information, and all are due to 
the same cause, which evidently is the difference of alloys as regards volatility. Thus 
in the case of gold and copper, the former being less volatile than the latter, there 
will be a minimum of metallic vapour with pure gold and a maximum with pure 
copper ; and when alloys are employed, there will be a regular gradation between these 
two limits. 
In the experiments about to .be detailed, unknown alloys and check pieces of known 
composition having been arranged on a suitable stand (as shown in the diagram, p. 496), 
they were in turn brought immediately under the fixed electrode F (of aluminium or 
iridio-platinum). 
We were soon convinced that it was necessary to regulate with extreme accuracy the 
length of the spark by which the incandescent vapour of the alloy was produced ; and 
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