ANALYSIS OP CEETAIN ALLOYS BY MEANS OF THE SPECTEOSCOPE. 497 
alloys termed by Matthiessen “ solidified solutions of one metal in another.” We next 
operated upon the gold-copper alloy, as its molecular arrangement appeared to render 
it peculiarly suited for the purpose of the research, the use of this alloy being attended 
with the additional advantage that the ordinary method of assay has rendered it possible 
to determine its composition with accuracy to the 10 q 00 part of the original weight of 
the assay-piece, a degree of precision which will appear remarkable to all who are 
familiar with the ordinary methods of quantitative analysis. 
We give in the accompanying curves (Plate XLI.) the results of our experiments with 
alloys of known composition, in which experiments the precautions to which we have 
referred have been taken. The results are represented graphically, the coordinates being 
the composition of the alloys as determined by the ordinary method of assay, and by the 
varying points of equal brightness as measured- on the micrometer-scale of the lines at 
the wave-lengths stated in the diagrams. The micrometer is fixed to the eyepiece end 
of the observing-telescope. It is an ordinary wire-micrometer with movable horizontal 
wires. The micrometer-readings are represented by spots placed opposite the compo- 
sitions as determined in the ordinary manner, a mean curve being subsequently con- 
structed. 
We have found that there are several variations possible in the method of observation, 
and we are yet uncertain which of these variations will afford the most satisfactory 
results. 
Our method of procedure was upon each occasion to prepare such a curve by 
means of accurately known standards ; and having this curve, to determine, by means 
of the micrometer-readings, the positions which various specimens of unknown compo- 
sition would occupy on it. By carrying the eye from the curve to the side on which, as 
in those communicated to the Society, the parting assay determinations were shown, 
we were enabled to compare the places assigned to the specimens, on the curve, 
by the spectroscope with those determined by the parting assay which we then learnt 
for the first time. We do not think it necessary to give these curves, but may say that, 
as the result of many series of experiments with alloys of unknown composition, the 
difference between the spectroscopic and parting assays was but small so long as the 
conditions under which we experimented continued to be uniform. 
Descriptions of Curves. 
Curve No. 1. 
A number of alloys of zinc and cadmium were synthetically prepared, and from these 
a series of five, the percentages of cadmium in which increased by 1 per cent, from 50 per 
cent, to 54 per cent., were carefully selected for the experiments. It is interesting to 
note that both the metals employed in the alloy are very fusible (zinc melting at 433° C. 
and cadmium at 228° C.) ; and observations by means of the spectroscope at once enabled 
us to arrange these alloys in their correct order. 
