Chemical Constitution of Saline Solutions. 255 
studied by Gladstone. E. J. Houston* has dealt chiefly with the spectrum 
observations of substances unaffected in chemical composition by change of 
temperature, as ferric oxide and cuprous iodide, &c. He concluded that heat 
caused the colour of the substance to pass from one of a greater to one of a less 
number of vibrations per unit of time. Similar phenomena were studied by 
Ackroyd, and termed metachromism or colour change.t 
A most important paper by Bunsen was published, in 1866, on the absorption 
spectra of salts of didymium.{ He examined the salts both in the crystalline state 
and in solution, and found that they presented several differences. The width of 
the absorption bands varies with the thickness and the quantity of salt contained 
in the absorbing medium. Solutions of the chloride, sulphate, and acetate, 
each containing the same quantity of didymium yielded different spectra, the 
bands being shifted towards the red with increase in the molecular weight of the 
salt. Drawings to scale are given, but it is to be regretted that the measurements 
were not reduced to wave-lengths. It may be mentioned that an instrument of 
great dispersion was found necessary to establish the fact that the bands were 
shifted, and that each of the bands near D, E, and F showed this displacement. 
The interpretation of this phenomenon, according to the views which I have 
expressed elsewhere, is, first, that the salts are not hydrolysed into an acid and a 
base; secondly, that chloride, sulphate, and acetate each exists as an integral 
molecule in the solution ; and thirdly, that as the molecular mass of the didymium 
salt is increased, its rate of vibration is proportionally retarded.§ 
Melde investigated the spectra of mixed coloured solutions, and studied also the 
action of heat on absorption bands.| 
H. Burger believed that the changes observed by Melde were not merely physi- 
cal, but partly chemical, taking into account the work of Magnus and H. W. Vogel.4 
Landauer showed that saffranin and its salts formed differently coloured 
hydrates in solution, and could undergo dehydration to a greater or less extent by 
the addition of more or less strong sulphuric acid to the aqueous solution.** 
W. J. Russell made a very careful examination of the salts of cobalt,++ in the 
fused state, and when mixed with other fused salts, in various indifferent solvents, 
in alcohol and acids. 
* Chem. News, vol. xxiv., p. 177. 
} Chem. News, vol. 34, p. 75, 1876; and Phil. Mag., vol. 2, p. 423, 1876. 
{ ‘‘ Ueber die Erscheinungen beim Absorptions-Spectrum des Didyms.’”’ Pogg. Ann., vol. 128, p. 100. 
§ Trans. Chem. Soc. vol. xxxix., p. 165, 1881. 
|| Pogg. Ann., vol. cxxiy., p. 91, and vol. cxxvi., p. 264. 
{| Spectroscopische Untersuchungen iiber die Constitution von Losungen. H. Burger. Ber., vol. ii., 
p- 6, 1876-78, 1878; Praktische Spectralanalyse irdischer Stoffe. H.W. Vogel, pp. 123 and 212. 
** Zur Kenntniss der Absorptions-Spectra. Ber., vol. ii., p. 1772. 
Tt Proc. Roy. Soc., vol. 31, pp. 51-54; and 82, pp. 258-272. 
2N2 
