REACTIONS OF STARCH. 21 



In order to see the direct action of the iodine, await its pene- 

 tration to some spot already selected and focussed upon this 

 spot, however, being chosen not too far from that part of the 

 edge of the cover-glass at which the reagent is placed and follow 

 by movement of the object-slide the progress of its action. We 

 see that immediately the influence of the iodine begins to make 

 itself felt, the starch grains stain, first pale, then bright blue, 

 and rapidly darken until they are black blue. At the first 

 moment of the reaction the lamination shows up clearly dis- 

 appearing as the grains become opaque. With potassium-iodide 

 iodine solution, in case this is added in considerable quantity, a 

 dark-brown coloration of the grain is quickly induced. Similarly 

 dry starch grains, which are exposed to the action of iodine vapour, 

 become a very deep brown. If we add water to such a prepara- 

 tion, the brown changes rapidly to blue. If the action of the 

 reagent does not proceed rapidly enough under the cover-glass, it 

 can be easily accelerated by sucking it through by a fragment 

 of blotting-paper placed at the opposite side of the cover. The 

 most beautiful violet-blue coloration of the starch grains is 

 obtained, however, when a scale of iodine is laid amongst the 

 starch grains in the drop of fluid under observation. The stain- 

 ing commences immediately in the vicinity of the scale. 



The blue reaction of starch, therefore, occurs only in the 

 presence of water. If air-dry potato or other starch be placed on 

 a slide, covered with a cover-glass, and a little alcohol iodine run 

 in, the grains are coloured a more or less deep yellow or reddish- 

 brown. If, however, a little water be now run in, the blue colour 

 at once appears. A similar appearance is presented if potato 

 starch in iodine, when heated for the purpose of swelling, is 

 heated to dryness. The grains, previously blue, become brown, 

 but the blue colour is restored on running in more water. If 

 iodised starch in small quantity is present in water, the blue 

 colour may fade and disappear on heating, returning on cooling. 

 This, again, is characteristic of the iodine reactions for starch. 

 We can stain with iodine the rod, etc., shaped grains of Euphorbia 

 also, in order to demonstrate that, in spite of their unwonted 

 form and scarcely noticeable lamination, these bodies are true 

 starch grains. 



Swelling (Gelatinisation} of Starch. The phenomena of the 

 swelling of starch grains under the influence of caustic potash 

 solution (potassium hydrate) may now be studied. We will once 



