Annual Report of the Council. Ixi 



he confirmed the work of Roscoe and Dittniar on the absorption 

 of ammonia by water, which had been criticised by Carius, 

 and he proceeded to shew that concentrated solutions of 

 ammonium nitrate and sodium nitrate absorb the same volume 

 of ammonia as pure water, and that the same amount of heat is 

 evolved when a given weight of ammonia is dissolved in various 

 solutions. 



It was as a fit reward to long, patient, and ingenious work 

 on many subjects that Raoult had the good fortune to discover 

 one of the most important laws in physical chemistry, and to open 

 up a new and most fruitful domain. Blagden was the first to 

 draw attention, in 1788, to the lowering of freezing points pro- 

 duced by the solution of a solid in water, and to show that, in 

 general, for dilute solutions the lowering was proportional to the 

 concentration of the solution. Riidorff, in 1861 and 1862, 

 re-discovered Blagden's law. In 187 1 L. C de Coppet extended 

 Riidorff s work, and shewed that the 'molecular lowering' was 

 the same for groups of allied salts in water. Raoult, who began 

 his work on the subject in 1878, studied the depression of the 

 freezing point not only in water but in other solvents, and 

 shewed that the aqueous solutions of inorganic salts, to which 

 his predecessors had devoted their attention, behaved exception- 

 ally. With most solvents he found that the depression of the 

 freezing point was the same for solutions containing the same 

 number of molecules of dissolved substance per litre, or, to use 

 an accepted phrase, that for a given solvent 'the molecular 

 depression ' was the same for all dissolved substances (1882). It 

 became possible by the new method to determine the molecular 

 weight of an immense number of organic compounds of which 

 the vapour density could not be obtained. His discovery of the 

 general law for dilute solutions was thus of great value and 

 importance not only to physical chemistry, but to chemistry 

 generally. But the discovery that the behaviour of water as a 

 solvent was exceptional and not normal proved perhaps of equal 

 importance. Arrhenius pointed out in 1887 that the abnormally 

 large depressions of the freezing point occurred only in the case 



