of Gibbs's Theory of Surface-Concentration. 523 
equation already obtained and calculate the adsorption co- 
efficient (T). 
Thus, taking as an example the solution of concentration 
O'l per cent., 
c = '001 grin, per c.c, 
dcr _ 8 dynes 
dc 
c do- 
Mdc 
•001 grm./c.c. 
■001x8x690 
289x2 x4*2xl0 7 x -001 
= 2*3 X 10~ 7 gram per cm. 
where 690 = the molecular weight of the undissociated 
Congo red, assuming it to be a monosodium derivative. Of 
course there is doubt as regards the value, since we are 
dealing with the substance when dissolved. 
So 
4s 
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Thus, assuming complete dissociation in solution, the 
value of the molecular weight would be 345, and the 
resulting value for the adsorption coefficient would be 
1*15 X 10 -7 grm. per cm. 2 On the other hand, if the dye is 
colloidal the molecular weight might possibly be much higher ; 
though the behaviour of sod. glycocholate, by analogy, is 
rather against this. 
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