APPENDIX 161 



Fix for six to twenty-four hours, wash in running water for 

 twelve to twenty-four hours. Preserve in alcohol 80 per- 

 cent. 



This is one of the standard killing mixtures, and for general 

 histological use it is excellent. It would be well to treat the 

 fixed tissue in alcohol with iodine for twenty-four hours to 

 remove all excess of the mercury salt. After this the iodine 

 must be thoroughly removed from the tissue by several 

 changes of alcohol. 



Tests for Organic Substances. 



Grape Sugar or Glucose. Into a test tube place the sub- 

 stance to be tested, add Benedict's or Fehling's solution and 

 boil. The appearance of a yellowish or red color indicates 

 the presence of the grape sugar. 



Starch. If a substance containing starch is acted upon 

 by an iodine solution a blue color will result. The reaction 

 takes place more quickly if the substance has been boiled 

 and the starch grains swelled. 



Cellulose. This substance is common in plant tissues, in 

 the fibers and cell walls; cotton is almost pure cellulose. 

 The section, or material, to be tested for cellulose is treated 

 with a drop or two of chloriodide of zinc. Cellulose is 

 colored violet, lignified membranes a yellowish brown, 

 membranes containing cutin or cork from yellow to yellow- 

 ish-brown. 



Fat. Fats and oils are stained black by osmic acid. A 

 1 per cent solution acting on a thin piece of tissue will show 

 the reaction within a few minutes. Sudan III in aqueous 

 solution will stain fat or oil a yellowish color, and Scharlach 

 R will give a red color. These stains will also act upon fat 

 preserved in formalin. Ether or benzene may be mixed 

 11 



