Connecticut western news., 1877-02-16
SCIENTIFIC.

The English Mint The Spectro- scope is now largely employed in the mint in London, for testing the fine- ness of the gold and silver. In the re-
 port by MT. Roberts. the chemist of the mint, diagram is given showing the comparative, results of spectroscopic assaying and the ordinary parting as- say. This diagram consists of d curve the coordinates of which are furnished by the indications of the micrOmeter in the eye piece of the spectroscope, to- gether with the known composition of several alloys. The chief point noted with regard to this curve is that there is much greater change in the micro meter readings for corresponding change of composition in the alloy in the less pure than in the finer alloys. Thus the difference between an alloy containing 916.7 parts gold in the 1000, and that containing 916.6, is shown by (roughly speaking from an inspection of the drawing) thirty divisions of the mlcrometer scale, whereas the differ ence between 916.3 and 9162 is shown by about 200 divisions. This latter fact may serve to show how infinitely more susceptible the spectrOscOpic me- thod IS than that Tn common use, when one ten thOusandth part Of alloy Is marked by so very considerable a change in the scale. It does not, how ever, appear that any great practical advance has been made in this novel application of the spectroscope, al- though on one occasion this instru- ment was used in testing the recently made trial plate of pure gold. It was found that its spectrum, when compar- ed with the solar spectrum showed ab- sOlutely no lines except those due to gold. The result of Mir. Roberts's long labOrs was thus approved by the most delicate of all possible tests. How long these labors were may be seen by ref erence to the report of the preceding year, in which it is stated that over 1000 assays were made in the prepara tiOn Ot the two gold plates, the standard -carats or 916.66 parts in the 10OO) and that absolutely pure. The neces sity for these plates arises from the inn possibility of obtaining absolutely ac- curate results from the usual process of chemical analysis. It has. there fore, long been the practice in the an- nual testing of the coinage to check the result by assaying, side by side with the alloy under examination, pieces of metal of known fineness. The test plates h use since 1899 were found not to be of their reputed standard, that for gold being actually 916.3 in stead of 016.6. It was, therefore, de- termined to prepare new plates, and this was done, with minute and elabor- ate care, as detailed in the report for 1873. To still further check the accur- acy of the result, plates of absolutely pure gold and silver were also prepar. ed, and 1t was this new pure gold that was examined by the spectroscope. These plates having been formally tested and approved in 1873, the an nual trial of the gold and silver coin age was effected by their aid in 1874. The natural result of this elevation of the standard was that the "Trial of tine Pyx" showed an apparent falling off in the fineness of the coins. However the greatest variation from standard was only four -thousandth parts (.O0O4) or one fifth of the legal "reme- as 8 limit for variation.