1909.] 



TJie Properties of Colloidal Systems. 



283 



T)f 1 n t,i rvr» ( — rtmnViPT* of 



litres containing 

 1 gramme). 



Number of particles 

 in 1 c.c. 



280 

 420 

 560 



14—16 x 10? 

 10 -7—12 -5 x 10 7 

 8 -9 x 10? 



Now, the total weight present in 1 c.c. of the original solution is. 

 4 - 65 milligrammes, so that the weight of each particle is 



- — = 2'3 x 10~ n milligramme. 



Further, the specific gravity of the solid acid is l - 46, determined by 

 weighing under toluene in a pyknometer.* Hence, the diameter of each 

 particle is 310 /jl/a. 



"We may, perhaps, go even further still. According to the osmotic pressure 

 measurements and assuming the kinetic origin of this pressure, each particle 

 contains on the average some 20 molecules ; so that, if this theory be 

 correct, we ought to be able to obtain an approximate value for the molecular 

 dimensions of this body. When calculated from the data given, the weight 

 of a molecule of congo-red acid comes out to be 



1"16 x 10 -12 milligramme, 

 or nearly 10 9 times that of hydrogen.f And the diameter 



111 fl[JU. 



The molecular weight being 652 372, the number of molecules contained in 

 1 gramme-molecule comes out as 



652-372 ...... 018 



= 5-6 xlO 1 



1-16 x 10- 15 



The number of molecules in a gramme-molecule of a perfect gas is usually 

 estimated at about 



6 x 10 23 .} 



Considering the many sources of error, the result obtained for the 

 molecular dimensions of our colloid does not seem very far out. This being 

 so, the hypothesis of the kinetic origin of osmotic pressure is, so far, 

 supported. 



The chief difficulty in the estimation of the number of particles under the 

 ultra-microscope is, in the case before us, the lively movements which they 



* Ostwald-Luther, ' Phys.-Chem. Mess.,' 2te Aufi., p. 147, 1902. 

 t Walker, ' Introd. to Phys. Chem.,' 4th ed., p. 217. 

 | See Pen-in, 1 Comptes Rendus,' vol. 147, p. 531, 1908. 



x 2 



