24: M. C. F. Schocnbein on the joint Influence exerted by Light 



of any free oxygen, has no appreciable action upon the indigo 

 solution ; for you may keep them together any length of time 

 (even in sunlight) without being able to perceive any diminution 

 or change of colour. So soon, however, as you permit oxygen 

 or atmospheric air to enter, an action will ensue, very slow in 

 the dark ; comparatively speaking, very rapid in solar light. 



The simplest way of exhibiting the action is this : suspend 

 moist strips of linen dyed with indigo solution in a mixture of 

 gaseous sulphurous acid and oxygen gas or atmospheric air. 

 When placed in the dark, such strips require many weeks to 

 become entirely white, whilst a strong noon^s insolation of one 

 and a half, or at most two hours' duration, will completely bleach 

 them. 



As a matter of course, the colour of aqueous sulphurous acid 

 mixed up with some indigo solution and shaken with oxygen 

 or atmospheric air, \vill also be much more rapidly discharged in 

 solar light than it is in the dark. 100 grammes of a weak sul- 

 phui'ous acid, coloured by 1 gramme of the standard indigo 

 solution, on being continually shaken with atmospheric air and 

 exposed to a strong sun, lost the colour within twenty minutes. 

 It is hardly necessary to mention, that the bleaching power is 

 exerted as long as there is free oxygen and sulphurous acid ; for 

 no sooner has the latter disappeared, i. e. been transfoi-med into 

 sulphuric acid, than the action ceases to take place in a per- 

 ceptible degree. 



I w^as curious to see how much of my standard indigo solution 

 could be destroyed by a given weight of pure sulphurous acid. 

 Five grammes of the latter, mixed up with 200 grammes of water, 

 were therefore put into a bottle holding about two litres and 

 filled with atmospheric air,, then 50 grammes of the standard 

 indigo solution added, the whole exposed to the action of solar 

 light, repeatedly shaken, and the atmospheric air now and then 

 renewed. There being little sunshine, the colour of the liquid 

 was discharged within a couple of days, and in the course of six 

 weeks (very deficient in sunshine) I have been able to destroy 

 600 grammes of the standard indigo solution, and find that there 

 is still a very perceptible quantity of sulphurous acid in the 

 mixture. The discharging power is therefore not yet exhausted. 



Now to discharge the colour of 600 grammes of my indigo 

 solution, I require nearly 1 1 grammes of the strongest nitric acid 

 (the monohydrate), or fully 18 grammes of good chloride of lime ; 

 and 5 grammes of pure sulphurous acid having already done the 

 same work, and being able to do still more, we see that sulphu- 

 rous acid enjoys a most extraordinary indirect oxidizing power. 

 I say " indirect,'' because the acid itself has nothing to do with 

 the oxidation of indigo; the only part it performs conjointly 



