ULTRAMICROSCOPY 83 



is certainly of great interest, and is a distinct advance towards 

 the solution of the problem of the Brownian movements. 

 Moreover, it furnishes a concise method of finding N , the 

 number of molecules in a gram molecule, a method the accuracy 

 of which can, apparently, be pushed to any limit we please. 



When the value of N is known, some other important 

 physical constants, such as the value of the atom of electricity— 

 the charge of a corpuscle or of an ion — can be readily deter- 

 mined. 1 



Electrical Properties of Colloids 



Towards the middle of the last century, Faraday discovered 

 that when an electric current passes through solutions of 

 different salts, the amount of an element deposited or liberated 

 in a given time is proportional to the chemical equivalent of the 

 element. Thus, when the same current passes through solutions 

 of silver nitrate and copper sulphate, the amounts of silver 

 and copper deposited, in the same time, are in the ratio 

 108 : 32*5. Later work, combined with this, has led to the 

 idea that the current in such a solution is carried by the ions 

 that are present. Thus in a solution of silver nitrate there are 

 ions of silver with a positive electric charge, and ions of the 

 radical N0 3 with a negative charge. Under the influence of 

 the electric force, these move in opposite directions ; the nega- 

 tive ions against the current, the positive with it. 



In the case of colloidal liquids there are many differences. 

 The conductivity of such liquids, when no electrolytes are 

 present, is very small ; and further, the colloid usually moves 

 as a whole, either towards the cathode or the anode. 2 The 

 former class, known as positive colloids, includes the mineral 

 hydrosols of solutions of metallic hydroxides, ferric hydroxide, 

 for example, and some dyes, such as night blue ; the latter, 

 known as negative colloids, includes many metallic hydrosols, 

 such as those of platinum, gold, and silver, and arsenious 

 sulphide. The movement of the colloid as a whole, towards 

 one or other of the electrodes, can be very simply shown by 

 employing Lodge's moving boundary method, or Whetham's 

 modification of it ; the surface of separation of the two liquids, 

 in the latter case, moves when a current passes. 



1 Electrolysis shows that Ne = 29 x io 13 electrostatic units. 



2 Some colloids — a neutral solution of serum globulin, for example — do not 

 move under the influence of an electric force ; they appear to be neutral. 



