( 4CI ) 



be incorrect by Dr. Cohen) that when placing tlie mixed, solidified 

 Ag Br-gelatine in water, there appeared in the latter Ag Br-flakes ; 

 he ascribed their formation to the diffusion of non-transformed K Br 

 and Ag NO3 from the gelatine into the water, where the reaction 

 then takes place and the ordinary flaky Ag Br appears. 



This opinion of Gaedicke's seemed improbable to Dr. Cohen; 

 and he has proved it to be incorrect by determining the conducti- 

 vity of the gelatine solutions before and after mixing. The reasoning 

 was simple; if the transformation is complete, the conductivity of 

 the gelatine solution (tested in a liquid slate at 30°) must be equal 

 to that of K NO3 ; if no reaction has taken place, the conductivity 

 should be greater and reach the total of the conductivities of A gNOs 

 and K Br in ratio to the imperfectness of the transformation effected. 

 The experiment decided in the former sense; the conductivity was 

 equal to that of K NO3 alone and the hardly visible transformation 

 had yet completely taken place ; the Ag Br is not dissociated into 

 ions, does therefore not conduct. 



So we have here before us some examples of phenomena that 

 must strike the chemist; the apparent non-appearance of transforma- 

 tions and of precipitation when gelatine is present in cases in 

 which a strong precipitation is observed in water alone. 1 have 

 furtlicr extended these observations over a long series of reactions 

 in which precipitation takes place. It has appeared then that what 

 has been observed in different cases under petiuliar circumtances for 

 purely aqueous solutions viz. the interesting phenomenon that some 

 bodies (silicic acid, titanic acid, chrome- and ferrichydrate, arsenious-, 

 antimony- and coppersulphide, silver, gold, selenium) can remain in 

 colloidal solution, is a rule for solutions in aqueous gelatine. 'J'he 

 colloidal medium keeps amorphous bodies which are created in it, 

 coUoidally dissolved. This holds good for instance for the halogenous 

 combinations of silver, the sulphides, and many hydrates of the 

 heavy metals, for metallic silver, gold and mercury, for chromate 

 of silver, Prussian blue, copper-ferrocyanide, iodide of lead, peroxyde 

 of manganese a. s. 0. They all give transparent gelatines which solidify 

 on cooling, preserve their transparency, but often show an internal 

 reflection or fluorescence. Especially for the coloured bodies, which 

 1 have cited here in the first place, the phenomenon is still remark- 

 able in so far as, by the very appearance of the color one need 

 not be in doubt about the question of the chemical change (as in 

 the case of Ag CI or Ag Br) and need therefore not convince oneself 

 of its having taken place by determining the conductivity. 



The modus operandi is nearly the same in all cases ; deviations 



