THE SO-CALLED INOEGANIC ENZYMES 229 



established under the distilled water. The electrodes consist of stout 

 platinum, gold, or silver wire from 1 to 2 mm. in diameter, and can 

 either be passed through two glass tubes so as to be easily handled, or 

 sealed into two glass tubes which are filled with mercury by means of 

 which electric contact is made with the platinum. 



In using the apparatus a short circuit is made between the two 

 electrodes under the distilled water, and then separated, when an 

 electric arc is established, and glows beneath the water; minute par- 

 ticles of metal are now detached from the negative pole, some of which 

 pass to the positive pole, but others remain in the distilled water and 

 form an exceedingly fine suspension or colloidal solution of the metal. 



The colloidal suspension begins at once, and in a few minutes a 

 fluid of a deep dark colour is obtained, which in the case of platinum 

 resembles the colour of reduced osmic acid solutions. Accompanying 

 the colloidal particles there is always a certain amount of metal in 

 coarser suspension, which may be removed by allowing to stand, filter- 

 ing, or centrifugalising. The dark-coloured solution remaining is 

 perfectly clear ; it may deposit a little more platinum in the first day or 

 two, but the rest remains in solution for months, and apparently in- 

 definitely. 



In the case of gold the colloidal solution has a deep dark-red colour, 

 while the silver sol varies in colour from dark reddish -brown to olive- 

 green, according to the dilution and fineness of the subdivision. 



All the metal sols are extremely sensitive to the presence of elec- 

 trolytes; they cannot be prepared in normal saline, and even addition 

 of normal saline to them when prepared precipitates them entirely 

 from solution. 



The catalytic reactions of metal sols which have been studied 

 are the conversion of hydrogen peroxide into water and oxygen, 

 and of hydrogen and oxygen to water. These have been most 

 studied in the case of the platin-sol by Bredig and his pupils. 



The action of platin-sol upon peroxide was found to follow 

 approximately the logarithmic law for a monomolecular reaction, 

 but in the case of hydrogen and oxygen Ernst found that the 

 velocity was proportional to the absolute amount of platinum 

 present, and in the peroxide experiments it was found that the 

 activity did not vary in simple direct proportionality to the con- 

 centration in platinum, but was expressed by the empirical law, 



k /C \ b 



A=(-J, in which Jc and k 2 are the velocity constants corre- 



#2 VI V 



spending to concentrations in platinum of C l and C 2 , and b, instead 



