384 J. a Blctke— Colloidal Gold. 



Bj comparing tlie amounts of the barium compounds thus 

 found with the amounts of the barium compounds held by 

 the filter in blank experiments similarly conducted, and with 

 the amounts of the barium compounds held by 1^'"^ of the 

 supernatant liquid, a very fair estimate of the amount of the 

 barium compounds held by the gold can be arrived at. In 

 these experiments the amounts of carbon left with the gold 

 after ignition, not readily dissolved in the subsequent treat- 

 ment with agiua regia^ first became noticeable, indicating that 

 in the former experiments most of the carbon had been lost by 

 decantation. In the experiments given under C (p. 387), a 

 freshly prepared solution of barium hydroxide, containing about 

 20^'"°' per liter, was used as the precipitant, the supernatant liquid 

 being still slightly acid ; while in the experiments given under 

 D, the barium hydroxide was added to alkaline reaction. The 

 wash-water was neutral to litmus. 



The results of Table II (pp. 386, 387) show that as long as 

 the solution is even slightly acid only insignificant amounts of 

 barium are held by the gold, whether washed or unwashed. 

 Other colloidal (blue) gold solutions, prepared by the action of 

 hydrazine hydrate on a water solution of gold chloride, in which 

 the free acid was nearly neutralized by the hydrazine hydrate 

 before the gold was precipitated by barium chloride, gave 

 similar results. When the supernatant liquid becomes alkaline, 

 on the other hand, the amount of barium retained by the gold 

 becomes appreciable. It is probable that none of this amount 

 was due to the presence of barium carbonate, although there 

 was doubtless some barium- chloride (formed by the reaction) 

 present with the hydroxide in experiment (12) of the table, 

 owning to the insolubility of the chloride in alcohol. There is, 

 however, no reason to suppose that tlie amount of barium 

 hydroxide retained by the gold p>recipitated from alkaline 

 solution is greater than that which would have been retained, 

 owing to the well-known tenacity of free alkalis for solid sub- 

 stances, by any finely-divided insoluble substance under similar 

 conditions of concentration." In fact, the absence of adsorp- 

 tion phenomena in acid solution and the presence of such 

 phenomena in alkaline solution is in full accord with Yan 

 Bemmelen's functional equation for absorption from solution 

 by porous solids 



C"kOH =/(Ck2S045 ^"sOa) 



where C^^koh is the concentration of the base in the liquid 

 retained by the solid, and C^ represents the concentrations of 

 the given substances in the supernatant liquid. Q" increases 

 as C'k2S04 increases, and decreases as C^sog increases. 



*Cf. Van Bemmelen, Zeit. anorg. Chem., xxiii, 364. 



