MICROCHEMIS TKY. 45 



or paraffine to prevent the evaporation of the fluid and the 

 access of oxygen. 



2. Peroxide of Hydrogen, H 4 <X- 



64. For testing living Spirogyrce for the presence of perox- 

 ide of hydrogen, Bokorny (III) used the two following 

 methods. 



The first is based on the fact that peroxide of hydrogen 

 in the presence of iron sulphate at once sets iodine free from 

 potassium iodide, so that any starch or starch-paste present 

 is colored blue. He therefore placed Spirogyra cells con- 

 taining starch in a very dilute solution of ferrous sulphate 

 .and potassium iodide, and deduced the absence of peroxide 

 of hydrogen from the failure of the starch-grains to become 

 colored blue. This was emphasized by the intense bluing 

 of the starch in threads which had previously been saturated 

 with the peroxide. 



In the second method, Bokorny acted upon the fact that 

 tannin which gives a blue reaction with ferric salts is at 

 once turned blue by ferrous sulphate in the presence of 

 peroxide of hydrogen, while the blue color otherwise appears 

 only after some time in consequence of the gradual oxida- 

 tion of the ferrous salt in the air. He observed, in agree- 

 ment with the above, that Spirogyra threads containing a 

 tannin that reacts with ferric salts became blue only many 

 hours after being placed in a solution of ferrous sulphate, 

 while the blue color appeared at once in threads saturated 

 with the peroxide. 



Pfeffer has (IV, 446) questioned the conclusiveness of 

 these experiments and especially doubted whether the dilute 

 reagents used by Bokorny were really taken up by the living 

 cells. But Bokorny has (I and II) recently made observa- 

 tions which show that the ferrous sulphate is really taken up 

 by the living cells, and the conclusiveness of the second reac- 

 tion cannot, therefore, be doubted. 



65. Pfeffer (IV) was led by more extended observations 

 to the conclusion that peroxide of hydrogen does not occur 



