318 HISTOLOGICAL TECHNIQUE. 



such as bone. Furthermore, some microscopical objects are obtained 

 suspended in fluids. In these cases special methods of preparation 

 must be used. 



1. Examination of Sediments. — For the collection of sediments 

 vessels — e. (/., test-tubes — with vertical walls should be used, or a 

 centrifuge may be employed, the sediment being removed from 

 the bottom of the tube with a clean pipette. Place the finger over 

 one end of the pipette before introducing it into the fluid, to retain 

 the air ; then bring the other end in contact with the sediment and 

 allow the air to escape slowly. Close the upper end of the pipette 

 and withdraw it. Carefully wipe the outside of the pipette, and 

 transfer the sediment to a slide and cover it with a cover-glass. 

 Many sediments may be stained with alum-carmine, methylene-blue,. 

 or Gram's solution, drained, and mounted in glycerin jelly. 



2. Microchemical Reactions. — These reactions are resorted to to 

 determine the chemical nature of objects under the microscope. 

 Every stain is the result of a microchemical reaction, but as yet 

 the knowledge obtained by staining tissues cannot always be ex- 

 pressed in chemical language. 



The manipulations are usually so conducted that the reaction can. 

 be directly observed under the microscope. The object to be studied 

 is placed in the middle of the field. The reagent used is then 

 placed at one edge of the cover-glass, whence some of it will flow 

 beneath the latter. To facilitate the entrance of the reagent a nar- 

 row strip of filter-paper may be brought in contact with the oppo- 

 site edge of the cover, withdrawing some of the fluid from beneath 

 it. It is best to sharpen the end of the strip which comes in con- 

 tact with the cover-glass, so that the absorption of fluid shall be 

 slow ; otherwise the currents induced will be likely to wash the 

 object from the field of vision. The following tests, applied in this 

 way, may be of use : 



a. Urates. Insoluble in 1 per cent, acetic acid ; soluble, on the 

 application of heat, in water (or urine). The slide must be removed 

 from the microscope when heat is applied to it. 



b. Earthy phosphates. Dissolve on the addition of 1 per cent, 

 acetic acid. Are not dissolved by heat. 



c. Calcium oxalate. Insoluble in 1 per cent, acetic acid ; soluble 

 in 1 per cent, hydrochloric acid. 



(1. Carbonates. Soluble in 1 per cent, acetic acid or hydrochloric 

 acid, with evolution of gas-bubbles. 



