TO MICRO-ORGANISMS AND DISEASE. [CHAP. 



facilitates the staining of the bacteria ; occasionally, also, 

 the addition of a few drops of liquor potassae. All sections, 

 after having been sufficiently stained, are transferred to and 

 washed in water, then methylated spirit, then in absolute 

 alcohol, then clarified in clove-oil, and finally mounted in 

 Canada-balsam (dissolved in chloroform, or better still in 

 benzol) or in Dammar varnish. 



In order to bring out by the dye more conspicuously the 

 bacteria present in fluids or tissues various methods are used, 

 all of which are based on the principle that the bacteria have 

 an affinity to the dye which is greater than that of the tissue- 

 elements. Hence after staining, the tissue-elements may 

 be decolourised without abstracting the colour from the 

 bacteria. Cover-glass specimens or sections, after having 

 been well stained with a dye, are subjected to various 

 decolourising re-agents, whereby the tissue-elements become 

 deprived of the dye, but the bacteria retain it. Although in 

 some instances this is not easy of achievement, since by 

 such decolourising processes also the bacteria are liable to 

 lose the stain, it nevertheless is possible in the majority 

 of instances. In many cases prolonged washing in alcohol 

 absolutus and in clove-oil is sufficient to abstract the dye 

 from the tissue-elements, but in some special cases, owing to 

 peculiar chemical properties possessed by certain bacteria, 

 the decolourising process requires special methods. Of these 

 the following are the most useful : 



i. In some instances the specimens (cover-glass specimens 

 and particularly sections) are stained in one dye, then 

 washed in alcohol till quite pale, then transferred to a 

 contrast dye. As contrast dyes are to be regarded blue 

 and red, or red and brown, or blue and brown, or violet 

 and brown. In some cases only the bacteria retain the first 

 dye, the tissue-elements become stained by the second dye. 



