412 THE COLLOIDAL STATE 



2. Both platinum hydrosol and enzymes gradually de- 

 compose spontaneously or decompose more rapidly by heating. 



3. There is an optimum temperature for both colloidal 

 platinum and for enzymes to exert their catalytic action. 



4. The activity of platinum hydrosol may be stimulated 

 by the addition of alkali until it reaches its maximum value, 

 after which the further addition of alkali causes it to fall again. 

 Similar stimulation of enzymes by the addition of certain sub- 

 stances known as Zymo-exciters have been observed in case 

 of emulsin acting on hydrogen peroxide, and of invertase 

 acting on cane sugar. 



5. The decomposition of hydrogen peroxide whether by 

 platinum hydrosol or by catalase, an enzyme contained in 

 blood, is in accordance with the laws governing a monomo- 

 lecular reaction. 



6. A very remarkable analogy between platinum hydrosol 

 and the enzyme of blood is that small quantities of substances 

 which, when added to the colloidal platinum solution, destroy 

 its catalytic action on hydrogen peroxide, also have the same 

 effect on the oxidase of blood. Curiously enough many of 

 these substances are blood poisons such as sulphuretted hydro- 

 gen, hydrocyanic acid, carbon monoxide, and arseniuretted 

 hydrogen ; several other substances were also found to para- 

 lyse either the platinum solution or the enzyme. 



It was further observed that platinum hydrosol when 

 treated with very small traces of hydrocyanic acid was tem- 

 porarily poisoned but recovered after a short time ; a similar 

 effect has also been observed with enzymes. The recovery is 

 probably due to the oxidation of the hydrocyanic acid. 



It was also found that the toxic effect of the hydrocyanic 

 acid was much greater if added directly to the platinum 

 or gold sol than if added to a sol already containing some 

 hydrogen peroxide. Exactly similar conditions had been 

 previously found by Schonbein * to hold in regard to the 

 addition of hydrocyanic acid and hydrogen peroxide to 

 blood. 



In conclusion, it should be noted that Bredig, while dis- 



* Schonbein : " Zeit. f. Biologic." 1867, 3, 144. 



