ON THE ULTRA-VIOLET SPARK SPECTRA. 277 
Several examples could be quoted of the analysis of minerals made 
by the spectroscope, the metallic constituents being estimated quantita- 
tively with exactitude and great facility. In some cases the results 
obtained by the spectroscope inspire greater confidence than those made 
by ordinary methods. 
The descriptive tables which follow are intended to be used with 
maps drawn to the scale of wave-lengths, and to a scale of actual mea- 
surements taken from photographs, so that the lines may be readily 
identified. The scale numbers given in the tables in hundredths of an 
inch refer to photographs such as those published in the ‘ Journal of the 
Chemical Society’ (‘ Trans.’ vol. xli. p. 90, 1882), from which actual 
measurements may be taken with an ivory scale. 
The limit of sensitiveness of the spectrum reaction is perhaps the 
greatest in the case of magnesium; one part of the metal in 10,000 
millions of solution is easily detected by the appearance of the lines with 
wave-lengths 2801°6 and 27941 attenuated and shortened. By increasing 
the strength of the spark the sensitiveness may be magnified 10,000 fold. 
It was shown in the Report presented in 1883 how spectrum observa- 
tions may be applied to determining the atomic weight of an element. 
Taking into account the spectrum of beryllium, this metal could find no 
place among the triad elements, but naturally took a position at the head 
of the dyad group. According to the periodic law its atomic weight 
would thus have the value 9. This view was opposed at the time, but it 
is satisfactory to learn that it has since been completely confirmed by the 
experimental work of Messrs. Nilson and Petterson and Dr. Humpidge. 
The Zine Spectrum. 
Wave-lengths 
Scale numbers 
1 per cent. 0-1 per cent. 0-01 per cent. 
Hundredths of an inch 
108-49 3344-4 3344-4 3344-4 
113°75 3301°7 3301°7 Ba0L:7 
116°30 3281°7 3281-7 32817 
145°69 3075°6 3075°6 
194:98 2800-1 2800°1? 
252°31 2557°3 2557°3 
267-95 2501°5 2501°5 
The Thallium Spectrum. 
Wave-lengths | 
Scale numbers 
1 per cent. 0-1 per cent. 0-01 per cent. 
Hundredths of an inch 
64°55 3775°6 3775°6 3775°6 
88:7 3518-6 3518°6 
143:0 3091-0 3091:0 
172°21 2917-7 
201°87 2767°1 2767°1 27671 
259°86 2530°0 2530°0 
335:27 2299°3 2299°3 
