88 HartLery—An Investigation of the Connexion between 
element, and not that of a compound, is evident in the case of silver, from the 
following facts :— 
Ist. The band spectrum is easily produced directly from the metal in the 
oxyhydrogen flame. 
2nd. It is not an oxide spectrum, because silver oxide cannot exist at such a 
temperature, and the same spectrum has been obtained by Basquin by means of 
the are in an atmosphere of hydrogen. 
3rd. It is not.a hydride spectrum, because silver hydride is decomposed at 
440°, and the spectrum can with equal facility be photographed from metallic 
silver heated in the oxygen and carbon monoxide flame. Another method of 
obtaining such spectra is that of Gouy, improved by de Watteville, by burning a 
mixture of coal-gas and air under pressure, the air conveying spray from a 
solution of a salt into the interior of a number of small jets of flame which 
burn with a green cone and a blue mantle. De Watteville has photographed 
the spectra of many elements produced in this manner,* but he mentions that 
with some metals, the flame spectrum contains bands of a more or less 
complicated character, in addition to the lines, and he has tabulated only the 
lines. 
In the paper ‘‘On Homologous Spectra” it was pointed out that Sir Norman 
Lockyert had proposed to explain the occurrence of coincident lines in the 
spectra of different elements by supposing that each spectrum is composed 
of several spectra, and that these compound spectra are the spectra of compound 
bodies, and not of elements; but as this hypothesis is scarcely compatible with 
homology in the structure of spectra, belonging to certain well-defined groups of 
elements in the periodic system of classification, it was suggested that, instead of 
single coincident lines, homologous groups of lines should be sought for. With 
this object in view the wave-length measurements of about 2,500 lines in the 
spark spectra of the elements{ were closely examined. The majority of the 
substances had been prepared especially in a state of great purity, and twelve 
of those concerning which any doubt was entertained underwent a special 
examination. The result led to the detection of similarly constituted groups of 
lines in some of the spectra (copper and tellurium, for example), which were 
subsequently shown to have their origin in impurities in the elements. It was 
also observed that by the method of experimenting employed, the lines of 
impurities, when present in small proportions, appeared at one electrode only, 
and were therefore easily recognized and identified. 
* Phil. Trans., 1904, 204, 189-68. } Proce. Roy. Soc., 28, 157, 1865. 
| Hartley and Adeney, Phil. Trans., 1884. 
