8 REAGENTS. 



starch will be colored a dark brown, and in the presence of an 

 absolute alcoholic solution of iodine, the same color is secured, but 

 if water is present, the color will be blue. This forms a ready test 

 for the presence of water in alcoholic solutions. Cellulose mem- 

 branes are colored by iodine a pale yellow or deep brown. If a 

 mixture of two parts of sulphuric acid and one of water be added 

 just before or after the test, the reaction will give a blue color, 

 while lignified cells if present will turn brown. 



Iodine kills protoplasm quickly coloring it a deep brown. 

 Alcoholic solutions of iodine deteriorate by standing, due to the 

 formation in them of hydriodic acid. The deterioration is greatly 

 augmented by the action of light and the solution should therefore 

 be kept in the dark. Two solutions of different strengths should 

 be in the laboratory, since it is important that the correct strength 

 be employed in the different tests. 



Millon's Reagent. 



This reagent is used for the detection of albuminoid sub- 

 stances, it readily causing them to turn a strong red color. The 

 reagent is prepared by pouring over some pure mercury an equal 

 quantity by weight of strong HNO a . If the solution is not com- 

 plete, heat the mixture, then pour over it twice its volume of water. 

 After allowing it to stand a few hours, decant the clear portion for 

 use. The reagent will act only when used in fresh solutions. 



Nitric Acid. 



Nitric acid is used with potassic chlorate as a macerating 

 agent. When added alone to tissues, it causes the protein matters 

 to turn a bright yellow. The reaction is made more apparent upon 

 the addition of ammonia. According to Hoehnel, the acid forms a 

 good test for suberin. It is also used for clearing tissue of starch, 

 causing the grains to swell and soon dissolving them. 



Oxalic Acid. 



An alcoholic solution of the acid is very useful in bleaching sec- 

 tions that have previously been too deeply stained. Dilute aqueous 

 solutions are employed with some stains for various tissues, and a 

 concentrated solution dissolves pectose after treatment with potash. 



