1875.] The Spectroscope in Mint Assaying. 87 
as of the eye, to detect any difference whatever. And, 
although the attempt was made to map an apparent difference 
between alloys varying two-thousandths, it would certainly 
not have been a safe test on which to base an assay. Fre- 
quent repetitions with changes of adjustment were tried, the 
battery power varying from one to six Bunsen cells, in con- 
nection with Leyden jars varying from one very small jar 
(improvised out of a test-tube) to fifty large jars, representing 
a metallic superficies of many square feet, with variations 
of the distance of the electrodes apart, and with and without 
the use of a condensing lens; but all these failed to give 
closer results. 
It is true that these changes of conditions produced cer- 
tain variations in the effects observed,—as, for instance, it 
was noticed that an increase in the Leyden jar surface 
always lengthened the lines, the distance between the 
electrodes and all other conditions remaining the same,— 
while a decrease in the condensing surface had an opposite 
effect. Thus, to take the extreme cases, with the single 
small Leyden jar above referred to, and one cell of battery, 
the lines broke when the electrodes were not more than one- 
sixteenth of an inch apart, and disappeared entirely on 
_ separating the points one-eighth of an inch. 
With fifty Leyden jars and six cells of battery it was 
found impossible to break the lines at all, even by removing 
the electrodes to the extreme limit of the spark, and in this 
case new lines also appeared. 
Other variations occurred—such as a momentary irregu- 
larity in the length and brightness of the lines, under a 
strong battery power, owing to the unequal action of the 
spark ; a difference in the action of the gold lines dependent 
upon the nature of the alloy, silver tending to lengthen them 
more than equal admixture of copper; the length of the 
lines is also dependent upon the distance between the spark 
and the slit (when the latter is used without the intervening 
condensing lens) ; moreover, the eye itself is liable to be- 
come confused by continued comparisons of very slight 
differences. The above, and other modifications, so far from 
solving the problem of close work, rather indicated possible 
sources of error. 
Another element of the process suggested itself as likely 
to render the results uncertain for the practical purpose of 
assaying, viz., whether the quantity of metal vaporised, and 
giving the spectrum, is not too infinitesimal to give safe 
results for a large melt. This would be affected by the least 
want of homogeneity in the metal. This is a serious 
