ON OUR KNOWLEDGE OF SPECTRUM ANALYSIS. 127 



II. Belation of the Spectrum of an Element to tJiat of its Compounds. 



There is perhaps no other inrestigation connected with molecular 

 vibrations which is of greater intei-est than that which tries to trace the 

 connection between the spectrum of an element and that of its compounds. 

 It was at first considered, as has already been mentioned, that an element 

 preserved its spectrum when entering into combination, so that, for 

 instance, the oxide of a metal would only show the metallic lines except in 

 so far as oxygen lines might be visible. This idea had to be given up, but 

 the absorption spectra of fluids were considered at first to be evidence in 

 favour of the assumption of permanence of the spectrum of an element 

 when combined with others. AV"e owe the first systematic investio-ation 

 on this point to Dr. Gladstone,' who examined the absorption spectra of 

 the solution of salts, each constituent of which was coloured. He came 

 to the conclusion that, generally, but not invariably, the following law 

 held good : ' When an acid and a base combine, each of which has a 

 difierent influence on the rays of light, a solution of the resultino- salt 

 will transmit only those rays which are not absorbed by either, or in 

 other words, which are transmitted by both.' 



Thus, for instance, chromic acid in solution cuts off the more re- 

 frangible half of the spectrum, admitting only the blue rays near F in 

 thin solutions ; but they transmit the less refrangible half perfectly. 

 This characteristic absorption of chromic acid remains when the acid 

 is combined with such bodies as copper, nickel, ferric oxide, uranium, 

 potash, and chromium ; but the salts formed by combination with the 

 three first-mentioned bodies show also their own influence when combined 

 with chromic or other acids by cutting off, as in copper and nickel, part 

 of the red end of the spectrum, or extending, as in ferric oxide, the' blue 

 absorption into the green. The characteristic absorption bands of uranium 

 salts are in the blue. As the chromic acid cuts off the blue, chromate of 

 uranium will not show the bands, but only a general absorption in their 

 stead. The potassium salts are colourless when combined with colourless 

 acids, and chromate of potash shows therefore the same spectrum as 

 chromic acid. Chromate of chromium forms an exception to the rule 

 for though the absorption peculiar to chromic acid exists, the absorption 

 visible in ordinary chromium salts does not appear. Soret^ has confirmed 

 Dr. Gladstone's conclusions with regard to the identity of the absorption 

 spectraof different chromates. The chromates of sodium, potassium, and 

 ammonia, as well as the bichromates of potassium and ammonia, were 

 found to give the same absorption spectrum. Kor is the effect of these 

 chromates confined to the blocking out simply of one end of the spectrum, 

 as in the visible part, but two distinct absorption bands are seen, which 

 seem unchanged in position if one of the above-mentioned chromates is re- 

 placed by another. These absorption bands show themselves only in weak 

 solutions ; the centre of one has a wave-length of about 3610 10"'" metres, 

 while the other is wider, and reaches from 2950 to 2440 approximately in 

 solutions containing about 0-1 grammes to the litre. From 2220 onwards 

 the spectrum is completely blocked out. Chromic acid itself showed the 

 bands, but less distinctly, and Soret does not consider the purity of the acid 

 sufficiently proved to allow him to draw any certain conclusions from this 

 observation. 



' Phil Mag. xiv. p. 418 (1857). 



- JiiMwthiquL' rnivemdle Arch. So. Ph. Ixi. p. 322 (1878). 



