Composition of so))U' Colloidal Solufions. 1^77 



increastnl the transpiration ^•alu(' so that the two bccanio 



ahnost identical, e. <t. : — 



7; before boilinu'. r; after boiling*-. 



Bv disk . . . 0-2278 ' 0-2032 



By transpiration 012r)0 O'l^al 



To exphiin these resnlts, consider the eiiect of setting a disk 

 in slight osciUation within a liqnid of the constitntion assumed 

 by Quincke. The colloidal cells which have fixed themselves 

 upon the disk are carrietl by it through the liquid and come 

 in contact with new cells, so that the mass of cells hangino- 

 on to the plate increases and with it the resistance to vibra- 

 tion, or, in other words, the logarithmic decrement. If the 



... 

 oscillation is too large the cell-walls will be drawn out and 



become thinner ; the thickness of the wall may become less 

 than twice the distance of the action of molecular force (2/) 

 in which case the surface-tension becomes less. The thick- 

 ness of cell-wall may even become zero and the cells then 

 tear themselves entirely away. Hence after a large oscilla- 

 tion the damping will be less than after a small one. 



Below^ the temperature tu the richly colloidal solution is 

 separating slowly out of the liquid as a liquid precipitate. 

 Its water content gets gradually less, and it finally goes 

 solid. The solidification is liastened by the addition of solid 

 jelly as described. 



Continued boilino- destrovs the cells and a gelatine solu- 

 tion then behaves as a homogeneous fluid. 



In the case of silicic acid and albumen the cell-walls 



appear to be much less easily torn. Stretching them, so 



long as they remain thicker than 2/, does not increase the 



surface-tension. 



On long standing the two constituents of silicic acid be- 

 ... 

 come by liquid precipitation more and more imlike, and 



the surface-tension — invohing the viscosity — proportionately 



greater. During a large oscillation of the disk in these 



solutions some of the cells are brought into contact with the 



disk and with other cells hanging upon it. They run together 



and increase the area of the surface of colloidal liquid in 



contact with the disk and thereby the surface-tension. Hence 



the decrement after a large swing is greater than after a 



small one. 



Boiling the sihcic acid has the effect of breaking up the 



cells, thus increasing the surface-area and surface-tension so 



that 7] appears greater after boiling. Mechanical shaking {in 



vacuo) has the same effect, ^^loreover, boiling drives out 



some of the air dissolved in the cell-walls and, as is well 



