3IO REAGENTS AND PROCESSES 



India Ink. — The gelatinous sheath of the conjugatas may 

 be demonstrated by placing the alga under investigation in 

 water in which India ink has been rubbed up until the water 

 has a dark gray color. In this the gelatinous sheath becomes 

 sharply demarked. 



Infiltration.— For infiltration with glycerine gum see page 

 306, with paraffin, page 261, with celloidin, page 266. The 

 stony tissues of seeds, etc., which are too hard and brittle to 

 be sectioned with a knife, and must, therefore, be ground to 

 the requisite thinness on a stone or by means of emery powder, 

 may be protected against breaking during this process if fairly 

 thin sections are first cut with a fine saw and then placed in 

 a rather thin solution of Canada balsam or copal in chloro- 

 form, which is then allowed to evaporate to the thickness of 

 syrup; the sections are allowed to dry and are then cemented 

 by means of a thick solution of gum arable to a glass plate 

 preparatory to grinding. Only a thin layer of gum arable 

 should be used, and this should be quite dry before the grind- 

 ing is begun. The sections may now be ground thin on a clean, 

 dry Arkansas or Wichita stone. Before the section has been 

 brought to the desired thinness, the surface should be polished 

 by rubbing it on a piece of soft leather which has been dressed 

 with tripoli. The stone on which the sections are ground may 

 be cleaned of the balsam from time to time by means of a cloth 

 dipped in xylol or turpentine. When one side has been polished, 

 the section may be freed from the glass plate by soaking in water, 

 and then the polished side should be cemented to the glass plate 

 and the reverse side ground and polished as before. The sec- 

 tions should be examined from time to time with the microscope, 

 so that the process of grinding may be stopped as soon as the 

 desired transparency has been obtained. They may then be 

 washed from the glass plate with water, and after drying should 

 be stained with fuchsin and mounted in Canada balsam. 



Iodine. — The fumes from heated crystals of iodine serve 

 well in many cases as a fixative. Small objects in drop cultures 

 may be readily fixed by pouring over them the fumes arising 



