Chemical Constitution of Baline 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. f 



A most important paper by Bunsen was published, in 1866, on the absorption 

 spectra of salts of didymium. J 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 diffei-ent 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. Bm-ger 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.^ 



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,tt 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. 



t Chem. News, vol. 34, p. 75, 1876; and Phil. Mag., vol. 2, p. 423, 1876. 



} " TJeber die Erscheinnngen beim Absorptions-Spectrum des Didyms." Pogg. Ann., vol. 128, p. 100. 



§ Trans. Chem. Soc. vol. xxxix., p. 16.5, 1881. 



II Pogg. Ann., vol. cxxiv., p. 91, and vol. cxxvi., p. 264. 



^ Spectroscopische Untersuchungen uber die Constitution von Losungen. H. Burger. Ber., vol. ii., 

 p. 6, 1876-78, 1878 ; Praktische Spectralanalyse irdischer Stofle. H. W. Vogel, pp. 123 and 212. 

 ** Zur Kenntniss der Absorptions-Spectra. Ber., vol. ii., p. 1772. 

 tt Proc. Key. Soc, vol. 31, pp. 51-54; and 32, pp. 258-272. 



2N 2 



