STAINING AND MOUNTING OF MICRO-ORGANISMS 83 



a very strong affinity for bacteria ; whereas the acid coal-tar 

 dyes, such as magenta, eosine, safranine, picric acid, and 

 the natural dye stuffs, such as logwood and cochineal, do 

 not possess this property. 



Gentian violet and fuchsine are the two dyes most 

 frequently used for the staining of bacteria. These stain 

 quicker and more intensely than any others. In order to 

 increase the staining properties of the dyes, certain reagents 

 are added to the stains to act as mordants. Phenol, aniline 

 oil, and alkalies are amongst the bodies most frequently 

 employed for this purpose. A very large number of stains 

 and staining methods have been devised by various workers 

 from time to time, but we will only give a few of the most 

 approved methods which are applicable to all ordinary 

 purposes. 



Stock solutions of concentrated alcoholic solutions of 

 stains, such as gentian violet, fuchsine, and methylene 

 blue, are prepared by allowing a large excess of the dye to 

 digest for some time in strong alcohol, shaking the solution 

 from time to time. The concentrated solutions are then 

 filtered and preserved in stoppered bottles. Most organisms 

 can be stained by means of a simple aqueous solution of 

 the dye, prepared by the addition of a few drops of one of 

 the above concentrated alcoholic solutions to water in the 

 proportion of about 1 to 4. Care should be taken not to 

 have the staining solution too strong, as it is very easy to 

 overstain. The solution can be tested as follows : The dye 

 should be of such a strength that ordinary print is just 

 visible on placing a watch-glass full of the stain upon some 

 ordinary printed matter. This, of course, only holds as a 

 general rule, as a stronger or a weaker solution is some- 

 times required. 



The best results are obtained by the use of one of the 

 following solutions : 



