1875.] The Spectroscope in Mint Assaying. 81 
trodes, consist of an alloy of two or more metals, the auto- 
graph of each may be clearly read. 
Mr. Lockyer noticed, while studying these luminous 
autographs, that when he separated the metallic electrodes, 
causing the spark to leap a greater distance through the 
air, the spectral lines no longer continued to cross the entire 
field of vision, but certain of them broke in the middle, and 
upon further increasing the distance between the electrodes, 
the hiatuses in the spectral lines increased proportionately, 
but unequally with different alloys. As the proportion of either 
metal of an alloy is increased, cts lines lengthen, and con- 
versely with the lines of the other metal. Upon this dis- 
covery, Mr. Lockyer based the theory of a possible method 
of quantitative analysis. 
The spectroscope was known to be marvellously sensitive 
to the impression of these autographs, and it therefore ap- 
peared plain that could sucha method of analysis be reduced 
to a practical basis, its value would be immense in assaying 
metals used in coinage. For although the present modes 
of assaying precious metals have been brought to great per- 
fection, yet the process is slow and tedious, requiring many 
chemical operations and great delicacy of manipulation; 
and “there is something captivating in the idea of a deter- 
mination, as it were, by a flash of lightning, or in the 
twinkling of an eye, what proportion of gold or silver is 
present in any bar or coin.” 
It was with the hope of reducing this beautiful theory of 
Mr. Lockyer to practice that these experiments were under- 
taken. The investigation extended over a period of several 
months, the principal part of the work being conducted at 
the University of Pennsylvania, with the benefit of the ex- 
cellent apparatus and appliances afforded in the new college 
building—a privilege kindly extended by Prof. Barker of 
that institution. 
A powerful induction coil, reinforced by Leyden jars, in 
connection with a two-prism Browning spectroscope, was 
employed, and it was found possible, after repeated com- 
parisons of the spectra of different known alloys of gold and 
copper, to map the difference of fineness between specimens 
having respectively 500 and 750 parts of gold in 1000 of the 
alloy, and even to recognise the variation between coin- 
ingots 895 and goz fine. This variation, within 7oooths, 
was by no means marked, although it seemed probable that 
a more delicate adjustment of apparatus, and further ex- 
perience, would render the distin¢tion more decided. 
The spark, in passing through the air, also vaporises its 
VOL. V. (N.S.) M 
