C. Bar us — Colloidal Silver. 453 



nearly, making the diameter (*2r) of the pores smaller than the 

 wave length of violet light. Schneider showed however that 

 colloidal silver passes readily through such a septum whereas 

 the alcoholic precipitate fails to do so. The particles are 

 therefore respectively smaller and larger than the diameter* 

 given. If 10 cm be taken as the order of molecular dimen- 

 sions, the size in question is at least 1000 times as large, show- 

 ing the aggregates to consist of the enormous number of 10' 

 molecules, at least. There is thus an abundance of room for par- 

 ticles containing (say) 100 molecules to the aggregate, and 

 forming suspensions in water (colloids) in their general aspects 

 hardly distinguishable from true solutions. 



It is interesting to ask how great a pressure would force the 

 water out of a septum just large enough to let the particles of 

 the size in question (5xl0~ 8 cm) pass. It would take several 

 thousand atmospheres, and it is therefore quite impossible to 

 test finer septa like animal membrane to the extent in ques- 

 tion. Nevertheless if the attempt be made to grade porous 

 clay septa, prepared by successive vitrifications, by the method 

 given, I dare say that a range of mean diameters of pores could 

 be obtained, sufficient to answer many outstanding dimensional 

 questions! in relation to the colloidal state ; but one should be 

 prepared to exert pressures as high as 100 atmospheres. 



2. There is another point of view from which colloidal silver 

 invites treatment, this time in the solid state. Dr. Schneider 

 and I interpreted the high degree of insulation which we 

 detected in Carey Lea's metallic mirrors as an instance of the 

 allied behavior of non-coherent metallic matter, in general. 

 Wohler (quoted by Wernicke) showed this some 50 years ago; 

 but our references were chiefly directed to the recent work of 

 Auerbach \ and of Ed. Branly § the latter of whom in par- 

 ticular proved that non-conduction ceased when an electric 

 spark was passed through the column of powder. One therefore 

 readily calls to mind the startling results recently obtained by 

 Oliver Lodge | with his " coherer," and the question is perti- 

 nently asked whether solid colloidal silver, swept by a train of 

 electric surges will begin to show increased electric conduction, 

 as it does for instance under the influence of heat. 



* By the hydrodynamic method particles smaller then 2 x 10~ 6 were measured, 

 supposing the method vouched for. Ostwald's Zts., 1. c. 



f I may here call to mind the allied geological fact that hot water will pass 

 through porous rock in virtue of capillarity, in the face of enormous withstanding 

 steam pressures. 



% Auerbach: Wied. Ann., xxviii, p. 604, 18S6. 



S Branly: Phil. Mag., (5), xxxiv, p. 530, 1892. 



|| Lodge: Nature, vol. 1, p. 136, 1894. 



