140 Dr. T. Ewan. On the 



x = - [ A/3'2 c + 0-000064 0-008]. 

 200 C 



The agreement found between the numbers obtained from two so 

 very different properties as the absorption of light and electric 

 conductivity, is so striking, taking into account the very considerable 

 experimental error, that little doubt can be felt as to the substantial 

 accuracy of the hypothesis on which the whole calculation is based. 



Ferric Salts. 



The changes of colour which take place when the aqueous solution 

 of a ferric salt is diluted are so considerable that they appear to 

 offer a very suitable means of determining the amount and nature 

 of the decomposition which the salt undergoes. 



In 1856, Pean de St. Grilles,* made a study of the decomposition 

 which ferric salts undergo on heating their aqueous solutions. He 

 noticed the peculiar appearance of the solutions containing ferric 

 hydroxide, they being clear by transmitted, but apparently turbid by 

 reflected light. He was aware that the hydrate exists in the liquid 

 in a state of pseudo solution and uncombined with the acid present, 

 as the following remark shows : " II semblerait plutot que 1'acide 

 acetique exerce sur 1'hydrate une action purement mecanique en 

 divisant a 1'extreme les particules insolubles et simulant ainsi une 

 dissolution veritable." 



In 1861, Grahamf showed how the colloidal ferric hydroxide can 

 be prepared by dialysis, and described its principal properties. He 

 regarded it as being different from the soluble hydroxide obtained 

 by Pean de St. Gilles, by heating the solution of ferric acetate. 



Gunning,{ in 1869, Tichborne, in 1871, and Berthelot,|| in 1873, 

 made studies of the decompositions which occur both on heating the 

 solutions and on diluting them with water. Their results, however, do 

 not lend themselves to a calculation of the amount of decomposition 

 which has taken place. In 1871, Krecke^]" made a number of deter- 

 minations of the quantity of ferric hydroxide formed in solutions of 

 ferric chloride of different concentrations, and at various temperatures. 

 The amount of ferric hydroxide present was determined by precipitat- 

 ing it with sodium chloride. He found that the precipitate formed by 

 heating solutions of ferric chloride containing more than 4 per cent, 

 of the salt contained a variable amount of chlorine. The more 



* ' Ann. Chim. Phys.,' (3), vol. 46, p. 47, 1856. 

 f ' Phil. Trans.,' 1861, p. 209. 



J ' Aanteekeningen, etc., Prov. Utrechtscke Grenootschap, 1869, p. 14. 

 ' Koy. Irish Acad. Proc.,' 1871, p. 169. 

 || 'Ann. Chim. Phys.,' (4), vol. 30, p. 145, 1873. 

 ij" ' J. prakt. Chem,' (2), Vol. 3, p. 286, 1871. 



