CATALYSIS 3 



chemical changes take place in nature they may be compared 

 in the fir^t place with ordinary chemical changes, which can 

 be effected in the laboratory with a minimum of assistance 

 from external chemical or physical energy. Examples of 

 such changes are frequent in the category of so-called catalytic 

 actions. 



The little cigar lighter, a smoker's toy which is often to be 

 seen in tobacconists' shops, is a good illustration of the chemical 

 action brought about by catalysis. In this case the warmth 

 of the hand causes a little alcohol vapour to evaporate from 

 the metal box and to impinge on a small knob of spongy 

 platinum which acts as the catalyst. Its precise mode of 

 action is not fully known, but it greatly accelerates the rate 

 of combination of the alcohol vapour with the oxygen of the 

 air, with the result that the alcohol bursts into flame. Platinum 

 in a state of fine division, such as may be obtained, for example, 

 by soaking asbestos in platinum chloride and driving off the 

 chlorine by heat, is thus capable of bringing about a number 

 of changes at temperatures much below those at which they 

 would normally take place. If a thread of asbestos, covered 

 with platinum in the manner above described, is warmed 

 and then held in a stream of coal-gas escaping, for example, 

 from an unlit Bunsen burner, the platinised asbestos will 

 glow. 



A technical process of importance, viz., the manufacture of 

 highly concentrated sulphuric acid, consists in passing sulphur 

 dioxide (S0 2 ), obtained by burning pyrites or sulphur, together 

 with oxygen, or air, over heated platinum in a fine state of 

 division. The two gases then combine in accordance with the 

 simple equation : 



S0 2 + = S0 3 



This combination takes place only to an infinitesimal extent 

 without the presence of a substance like the platinum, which 

 acts as a catalyst. 



Bi 



