4 MICRO-CHEMICAL REAGENTS. 



ous solution, this condition is fulfilled ; but in case 

 a solution in absolute alcohol is used, water must 

 be added to the preparation. 



The well-known reaction, discovered by Stro- 

 meyer, is as follows : ' According to the strength 

 of the solution and the duration of its action, the 

 starch grains assume a more or less deep-blue 

 color, which vanishes when the preparation is 

 warmed, but reappears when it again becomes 

 cool, and is entirely destroyed by the action of 

 hyposulphite of sodium. Dry starch slowly turns 

 brown when treated with a solution of iodine in 

 chloroform or absolute alcohol, which renders it a 

 good test for the presence of water in alcohol. 

 According to Nageli, the starch grains are com- 

 posed of granulose diffused through a skeleton of 

 starch-cellulose. The former, only, is turned blue 

 by iodine, for after its removal (by digestion with 

 saliva at 45-55 C., or the action of pepsin, dias- 

 tase, organic acids, or the prolonged action of dilute 

 sulphuric or hydrochloric acid) the grains turn yel- 

 low or brown. 



Most cellulose membranes color yellow or 

 brownish with iodine, especially when a freshly- 

 made solution is employed. An exception, how- 

 ever, is afforded by the paraphyses and asci, the 

 hymenium of lichens, which often turn blue, like 

 starch. If, however, the cell-wall has been first 



1 Nageli: Die Starkekorner, Ziirich, 1858. Wiesner: Technische 

 Mikroskopie, 1857, p. 73. 



