2 Taylor, Production of White Ferrous Ferrocyanide. 



ordinary solution of ferrous sulphate reduces it so com- 

 pletely that a creamy white precipitate is produced on 

 the addition of potassium ferrocyanide. Or a few drops 

 of either solution will change the ordinary light blue 

 precipitate into white ; indeed they are such powerful 

 reducing agents that they will turn Prussian blue perfectly 

 white. 



If there is no appreciable excess of hydrosulphite 

 present with the white precipitate, it immediately begins 

 to turn blue at the top, where the air has access to it. 



The white precipitate provides an admirable way of 

 showing the presence of dissolved oxygen in ordinary 

 water. If water which has been previously well boiled, 

 to expel dissolved air, is added to some of it there is no 

 change ; but ordinary tap water turns it blue instantly. 

 To ensure success in this experiment there must not be 

 any excess of the hydrosulphite present. Probably this 

 reaction might be adapted ro the determination of the 

 amount <.A dissolved oxygen in water. 



