44 MICRO-ORGANISMS IN WATER 



other for the purposes of bacteriological technique, the 

 former being alone available for the more striking ex- 

 hibition of micro-organisms, whilst the acid colours 

 may sometimes be taken advantage of for the tinctorial 

 demonstration of other elements in the microscopic 

 .specimen. 



The basic aniline dyes in most frequent use for the 

 staining of bacteria are : Fuchsine or magenta, gen- 

 tian violet, methyl violet, methylene blue, Bismarck 

 brown. 



It is most convenient in employing these colouring 

 matters to prepare a saturated alcoholic solution of 

 each which can be kept in stock, and these stock solu- 

 tions are then diluted with about ten times their volume 

 of distilled water for actual use. It is not advisable to 

 prepare more than small quantities of these diluted 

 solutions at a time, as they do not keep their tinctorial 

 powers for very long, although in this respect methylene 

 blue forms an exception, as its diluted solution even is 

 remarkably durable. 



Magenta and the two violets possess the strongest 

 tinctorial properties, and are the colours in most con- 

 stant and general use ; but methylene blue, inasmuch 

 as it stains less intensely, is extremely useful in the case 

 of some bacteria, like certain sarcinre, which take up 

 the colour very strongly, and in which it is sometimes 

 difficult to avoid over-staining the preparations. In 

 fact this stain is particularly useful in revealing the 

 more detailed and delicate structure of micro-organisms. 



It has been found that the staining powers of these 

 aqueous alcoholic solutions may be very greatly in- 

 creased by the addition to them of certain substances, 

 for when used by themselves they are incapable of 

 colouring some bacteria as well as spores and flagella ; 



