SPECIAL METHODS. 22/ 



403. Concerning the microchemical relations of starch, it 

 may be first remarked that it is entirely insoluble in cold 

 water ; but in hot water it suffers first strong swelling, and 

 on longer boiling it passes completely into solution (paste). 

 Caustic potash also causes a marked swelling and finally 

 complete solution of the starch-grains. 



But its becoming blue with iodine is especially character- 

 istic of starch. This does not take place indiscriminately 

 with iodine solutions, for all solutions containing hydriodic 

 .acid or potassium iodide give it rather violet-brown tones. 

 A pure blue coloring of starch-grains is obtained by prepar- 

 ing an aqueous solution of iodine immediately before it is to 

 be used, by adding a few drops of an alcholic solution, which 

 keeps indefinitely, to a few ccm. of distilled water. The use 

 of dilute solutions is always to be recommended for coloring 

 starch-grains, as they often become almost black in concen- 

 trated solutions. Swollen grains, and even paste, have the 

 same power as unchanged ones of becoming blue with iodine. 

 This color disappears on heating, but reappears on subse- 

 quent cooling, without further addition of iodine. Caustic 

 potash always at once destroys the color by the decomposi- 

 tion of the iodine. 



404. For the recognition of very small quantities of starch 

 various methods have been given, which consist essentially 

 in swelling the starch-grains before the addition of iodine 

 and destroying the rest of the cell-contents. But in most 

 cases it is as well to place thin sections in a concentrated 

 solution of iodine and potassium iodide ; the deep black 

 starch-grains then come out sharply with high powers and 

 strong illuminations. 



405. In many cases, the sections may well be treated 

 according to the method of A. Meyer (II). After treatment 

 with a pretty dilute solution of iodine and potassium iodide, 

 and the removal of this reagent, a concentrated aqueous solu- 

 tion of chloral hydrate is added to the sections, which 

 destroys the other cell-contents and causes the starch-grains 

 to swell so that they appear brighter and usually of a 

 beautiful blue color. But it should be observed that starch 



