50 MIORO-ORGANISMS IN WATER 



colours (Unna ^), to which belong methyl violet, gentian 

 violet, and Victoria blue, the strong affinity of these 

 for iodine being, according to Unna, the cause of this 

 remarkable circumstance. 



Double staining is also resorted to in the case of the 

 spores of micro-organisms, which will not take up the 

 colouring matter in the usual way. Special methods 

 have to be devised for the exhibition of both these and 

 the organs of locomotion, or flagella, with which some 

 bacteria are provided ; these will be described later. 



Cover-glass preparations of micro-organisms. — Having 

 described the various stains which are in use for the 

 colouring of micro-organisms, we must now consider 

 the means by which such stained preparations may 

 be microscopically examined. In the first place, the 

 cover-glasses must be scrupulously clean, every trace 

 of grease being removed. To accomplish this suc- 

 cessfully they may be heated for some minutes in con- 

 centrated sulphuric acid, then washed with distilled 

 water and transferred to a mixture containing equal 

 parts of alcohol and ammonia, after which they should 

 be dried with a perfectly clean soft rag. The cover- 

 glass may finally, before use, be passed a few times 

 slowly through the flame of a bunsen, so as to effec- 

 tually remove every particle of grease. The glass slides 

 on which the cover-glasses are mounted should also be 

 thoroughly cleaned and freed from dust. 



As it is important in examining cultivations of 

 micro-organisms that there should not be too much 

 material on the cover-glass, it is best to mix a small 

 quantity abstracted on the point of a sterile platinum- 

 needle, with a drop or two of distilled water on a cover- 

 glass, and from this dilution to remove some by means 



^ Die Rosaniline und Pararosaniline. Dermatologische Studien, 

 4tes Heft. Hamburg and Leipzig, 1887. 



