254 Hartley — The Action of Heat on the Absorption Spectra and 



above title, one was a preliminary notice,* the other a detailed account of the 

 spectra of a number of metallic salts, together with the conclusions arrived at as 

 to their chemical constitution when dissolved in water. 



The bare conclusions were publislied in the Proceedings as an abstract of the 

 Paper, t 



The greater part of the memoir was withheld for a time merely for the 

 purpose of reducing arbitrary measurements of the spectra to wave-lengths, and 

 of inserting Fraunhofer's lines in the diagrams. It happened that in consequence 

 of an illness, and other circumstances which intervened, the experimental details 

 here recorded were put aside and overlooked, but the results obtained twenty- 

 four years ago have quite recently been confirmed in several particulars. 



The following passage a2:)pears in tlie original communication and is still of 

 some interest : — 



"When a substance is dissolved in water, it is at present an unsolved problem 

 what the exact constitution of the resulting liquid really is. If the salt be 

 anhydrous like common salt, it may possibly combine with a portion of the water 

 to form a more complex molecule, which in its turn is dissolved, or the whole of 

 the water may combine with the salt to form a still more complex liquid mole- 

 cule. In the case of this latter alternative, such solutions are chemical compounds. 

 On the other hand, if the substance is one in which water forms an integral part 

 of the molecule of the salt, it may be dissolved without the molecule undergoing 

 any chemical change, or the salt may be separated from its water of crystalliza- 

 tion, and be dissolved as an anhydrous or partially dehydrated substance." 



There are many observed phenomena connecting the absorption spectra of 

 saline solutions with the molecular constitution of the dissolved salts which have 

 not yet been published, and at the present time are perhaps of greater interest 

 than at any earlier date. 



Schoenbein,J von Babo, Schiff,§ and others have observed the darkening of 

 both solids and solutions by the action of heat.|| Gladstone's^f experiments show 

 rather that haloid salts are not decomposed by solution in water, as was formerly 

 supposed, and furthermore that cupric chloride in solution, on addition of further 

 quantities of water, forms differently hydrated compounds.** 



In 1857, Dr. Gladstone published his well-known Paper " On the use of the 

 Prism in Qualitative Analysis. "ft The effect of heat on coloured liquids was also 



* Proc. Eoy. Soc, vol. xxii., p. 241. f Proo. Roy. Soc, vol. xxiii., p. 372. 



X Jahresberiolit, 1852, p. 301 ; 1853, p. 312; 1857, p. 72 ; 1859, p. 53. 

 § Ann. Ch. Phar., ex., p. 203. 



II Gmelin's "Handbook of Chemistry," English edition, vol. 5, p. 337. 



f Jour. Cham. Soc, vol. 7, p. 211. ** See also Jour. Cham. Soc, vol. 13, p. 206. 1875. 



ft Jour. Cham. Soc, vol. x., p. 79 ; Phil. Mag., vol. xix. ; Proc. Roy. Soc, vol. ix., pp. 66-74, 1859 ; 

 Jour. Cham. Soc, vol. xi., pp. 36-40, 1859. 



