126 ESSAYS ON BACTEEIOLOGY. 



solutions are sometimes added certain substances 

 ■which increase their power of staining. Most of the 

 known bacteria maj' be stained by exposure for a few 

 seconds or minutes to a simple watery solution of one 

 of these anilin dyes. The process may be hastened 

 or intensified or made applicable to otherwise resist- 

 ent organisms by heating the solution. If the bac- 

 teria still resist the dye, it may be made to "bite" into 

 them by the addition of mordants, such as anilin oil, 

 carbolic acid, or caiistic potash, following the methods 

 used, in the dyeing of cloth. The most vigorous 

 staining effects are secured by hot solutions contain- 

 ing such mordants, and thtis only are certain bacteria 

 successfully colored. Spores are, as a rule, more re- 

 sistant to staining than the mature bacteria. A spe- 

 cial method of staining is that of Gram, in which 

 iodine dissolved in potassium iodide solution is used to 

 protect from decolorization certain bacteria which 

 have once been stained. The reaction of bacteria to 

 this method of staining having been determined, it 

 subsequently serves as an aid in identifying them. 

 Decolorizing agents, water, alcohol, and mineral acids, 

 are used when desirable, to remove an excess or all of 

 the dye from stained bacteria. All stained specimens 

 are washed in water before examining them. The 

 behavior of microbes under the influence of the 

 stronger decolorizing solutions is sometimes of much 

 significance in distinguishing them from others sim- 

 ilar in appearance. This is an essential feature of the 



