436 HISTOLOGICAL TECHNIQUE. 



8. 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. 



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

 acid, with evolution of gas-bubbles. 



e. Albuminoid granules. Become indistinct, and finally invisible, 

 on the addition of 1 per cent, acetic acid or 1 per cent, potassium 

 hydrate ; not blackened by osmic acid. 



/. Fatty granules. Not affected by 1 per cent, acetic acid or 1 

 per cent, potassium hydrate. Stained black or dark brown by osmic 

 acid. 



g. Starch. Stained dark blue to black by iodine solutions. Use 

 Gram's solution. 



h. Cellulose. Stained yellow by iodine solutions. If the water 

 be then removed and concentrated sulphuric acid introduced, the 

 color becomes blue. The walls of most vegetable cells are composed 

 of cellulose. 



i. Teichmann's test for haemoglobin. This test depends upon the 

 conversion of the haemoglobin or its derivatives into hsemin, which 



