8 MICRO-ORGANISMS AND DISEASE. [CHAP. 



some special instances, such as the bacilli of leprosy and 

 tuberculosis, double staining is required. With other 

 organisms, such as the bacilli of glanders or tuberculosis, 

 the washing is carried out, not with water but with acid 

 (acetic acid and nitric acid respectively). All the details 

 will be stated when dealing with these special organisms. 



The method extensively and successfully used for the 

 demonstration and preservation of microscopic specimens 

 of micro-organisms in fluids, as blood, pus, and juices, is 

 that of Weigert and Koch, which consists in spreading out 

 on a glass slide or cover-glass a very thin film the thinner 

 the better of the fluid (artificial or natural culture medium), 

 blood, pus, or juice, and drying it rapidly by holding it for 

 ten to twenty seconds over the flame of a spirit-lamp or gas- 

 burner. The most successful preparations are obtained 

 when the heating is carried on for such a time that the film, 

 having become opaque at first, rapidly turns transparent. 

 Several drops of the aniline dye to be used are then poured 

 over the specimen, and after remaining on it from five to 

 thirty minutes or more are poured off. 



The cover -glass specimen is then well rinsed with distilled 

 water, dried over the flame, and mounted in Canada-balsam 

 solution or Dammar varnish of course always bearing in 

 mind on which surface of the cover-glass the film has been 

 spread. If the film has been well dried in the first instance 

 washing in water is quite sufficient, but if the drying has been 

 insufficient a good deal of diffuse staining of the ground 

 substance has taken place, and then of course the cover-glass 

 specimen must be also washed in alcohol sufficiently long to 

 remove this undesirable staining, then washed in water, dried 

 and mounted. In some instances washing with alcohol 

 removes also the dye from the bacteria, but as a rule it is 

 better to first over-stain the cover-glass specimen, then wash 



