IV. Researches on spectrum photography in relation to new methods of quantitative chemical analysis.—Part I

Abstract

In the year 1878 I submitted to the Council of the Chemical Society a series of photographs executed with roughly constructed and imperfect apparatus, which showed variations in the number, length, and strength of the lines exhibited by some of the elements in the ultra-violet region. These photographs proved that by alterations in the exposure of the photographic plates certain impurities became visible in metals otherwise apparently pure, without any changes being noticeable in the spectra such as might be introduced by over-exposure. Thus by increasing the period of exposure three-fold the lines of iron were plainly seen in a spectrum regarded as that of pure aluminium. Gold obtained in as pure a condition as possible by parting gave evidence of the presence of silver. Two specimens of indium were examined: they both yielded the flame spectrum and the reactions usual with this metal; one of them showed no strong lines in the ultra-violet which could not be attributed to tin, lead, or cadmium; the other contained tin and. cadmium in such proportions that the lines of these metals were more prominent than those of indium. A third specimen, prepared by Professor Pichter, of Freiberg, yielded no spectral lines which could be attributed to any foreign metal. A striking fact was noticed which, as will be seen, is exemplified in the case of magnesium, namely, that it is not always what appears to be the strongest and longest lines which first make their appearance when an impurity is discernible; and it was considered that many observations were necessary to ascertain which are the most persistent lines in the spectra of the elements. It was proposed to extend these observations, but their importance at that time did not seem to be appreciated. Almost all varieties of spark spectra have since been investigated, with the object of applying spectrum photography to the purposes of chemical analysis. From time to time opportunities have occurred when the process which has been gradually developed and rendered practical has been advantageously put into operation for the solution of questions upon which it would have been difficult to arrive at a decision by other means. For instance, I employed photographs of spark spectra in an examination of the rare earth bases separated from the mineral rhabdophane, for testing the purity of certain cerium compounds, and for estimating the amount of beryllium contained therein (Journal of the Chemical Society, vol. xli., Transactions, 1882, p. 210). It may therefore be considered that the method is so far complete in detail that it may be with advantage described for publication, notwithstanding that it is manifestly capable of improvement.