228 THE SO-CALLED INORGANIC ENZYMES 



in connection with these metallic catalysts, the effects obtained 

 with them are of high interest in regard to the manner in which 

 a colloid in solution can. act as a catalyst. 



It was already known to Faraday that all porous bodies, and 

 especially certain metals, such as platinum, possessed the pro- 

 perty of absorbing large amounts of gases. The velocity with 

 which the gases are absorbed increases with the state of sub- 

 division of the metal, and is best seen in the case of platinum when 

 this is used in the finely divided form of platinum black. Faraday 

 pointed out also that this subdivision favoured the action of the 

 platinum black as a catalyst by increasing the local concentration 

 of the substances to be catalysed, and even showed that the action 

 of catalysis by platinum black of an explosive mixture of hydrogen 

 and oxygen was stopped by the presence of traces of carbon bisul- 

 phide or sulphuretted hydrogen. It is wonderful how closely this 

 view of Faraday comes to the modern view with regard to the mode 

 of action of a colloidal catalyst or enzyme. 



The catalytic action of finely divided metals was taken up by 

 Bredig and Miiller von Berneck, and tested in the case of platinum 

 and hydrogen peroxide. For this purpose, finding platinum black 

 difficult to subdivide and suspend in solution, they evolved the 

 ingenious method of obtaining the platinum in colloidal solution. 

 Colloidal solutions of metals had already been prepared by Carey 

 Lea by chemical means, such as colloidal silver by reduction of 

 silver nitrate by ferrous sulphate; but Bredig discovered the much 

 simpler method of detaching the metal from the negative electrode 

 by means of a high potential in distilled water. 



The process consists in establishing an electric arc between stout 

 metallic electrodes of the metal of which it is desired to make 'a 

 colloidal solution, in as pure as possible distilled water, which must 

 be maintained at a low temperature. The conductivity of the water 

 must be low, or otherwise electrolytic conductivity comes in and 

 destroys the process, and, further, the presence of electrolytes tends 

 to precipitate the colloidal metal. 



The process can be best carried out by utilising the electric lighting 

 mains (the usual 110 volt, constant current, circuit), and placing in the 

 circuit an ammeter, a flat crystallising dish containing about 100 c.c. of 

 pure distilled water which has previously been boiled to expel dissolved 

 carbon dioxide, and a variable fluid resistance which is regulated 

 to give a current of 4 or 5 amperes after the electric arc has been 



