308 REAGENTS AND PROCESSES 



of glycerine and 25 c.c. of methyl alcohol. Before using this 

 it should be allowed to stand until all precipitates have settled. 

 Sections are transferred from water into the stain, where 

 they remain for several minutes; they are then placed in a dish 

 of 70 per cent, alcohol, acidulated with a drop of HCl, and after 

 a minute they are rinsed in 95 per cent, alcohol, then in xylene, 

 and mounted in balsam. 



JHaematoxylin and Safranin. — Sections stained in safranin 

 and washed in water may be placed for a few minutes in Dela- 

 field's haematoxylin, where they are treated as described under 

 this stain. By this treatment lignified and suberized walls are 

 stained red and cellulose walls violet. 



Hanging-drop Culture. — A hanging-drop culture is useful 

 in the study of various microorganisms. Spin a ring of melted 

 paraffin on the slide the size of the coverglass to be used. Wash 

 the coverglass with soap and water, rinse it and rub it bright with 

 alcohol on a clean cloth, and sterilize it by baking in an oven. 

 Handle it thereafter with sterilized forceps. By means of a 

 sterilized glass rod or pipette transfer a drop of nutrient solution 

 (see under Nutrient Media) to the middle of the coverglass, and 

 to this drop transfer with a sterilized needle a very minute por- 

 tion of the culture or material to be studied. Invert the culture 

 over the paraffin ring and press it down firmly so that a tight 

 union is made. This is necessary to keep the drop from evap- 

 orating. If the paraffin is too hard to make a close union, put 

 a thin layer of vaseline over the ring before putting on the cover- 

 glass. The drop should of course hang free in the cell and not 

 touch the slide. Sometimes it is desirable to draw the drop out a 

 little over the coverglass with a sterilized needle, or even to 

 flatten it out entirely by placing over it a coverglass enough 

 smaller than the first not to touch the paraffin ring. 



The dealers furnish hollow-ground slides that are excellent 

 for hanging drop cultures. With these no paraffin ring is needed. 



Cultures prepared in this way can be studied at any time, 

 even with high powers, without disturbing them. 



Hardening Processes. ^The hardening of tissues is accom- 



