STAINING METHODS. 27 



as above described, but the writer has for a number of years made 

 his mounts of bacteria upon the glass slide, and believes that this 

 method has some advantages for every-day work. The thin glass 

 covers required when a preparation is to be examined with an im- 

 mersion objective of high power, are easily broken and often dropped 

 from the ringers or forceps. When the material to be examined is 

 spread and dried directly upon the glass slide, the operation is at- 

 tended with less difficulty and fewer accidents and the results are 

 quite as good. In this case the slide is held in the fingers during the 

 various steps in the operation of distributing, drying, and staining, 

 while the thin glass cover must be held in delicate forceps. 



Contact Preparations. When a dry and clean cover glass is 

 brought in contact with a colony or surface culture we may often 

 obtain a very pretty preparation, showing the bacteria in a single 

 layer, and preserving the arrangement, as regards growth, which 

 characterizes the species. Similar preparations may sometimes be 

 obtained from the surface of liquid cultures, when the bacteria grow 

 upon the surface as a thin film. The cover glass is to be gently 

 brought into contact with this surface growth, which adheres to it 

 and is dried and stained by the usual methods. 



Staining of the dried film is quickly effected by using an aqueous 

 solution of one of the aniline colors above mentioned. For general 

 use the writer prefers a solution of f uchsin, on account of the prompt- 

 ness of its staining action, and because, in preparations for permanent 

 preservation, it is not as likely to fade as methyleiie blue or gentian 

 violet. It is also a better color than blue or violet in case a photo- 

 micrograph is to be made from the preparation. 



It is best to keep on hand saturated alcoholic solutions of the 

 staining agents named, and to make an aqueous solution whenever 

 required by the addition of a few drops to a little water in a watch 

 glass or test tube ; for the aqueous solutions do not keep well on ac- 

 count of the precipitation of the dye as a fine powder, which ren- 

 ders the solution opaque. The addition of ten per cent of alcohol 

 to the aqueous solution will, however, prevent this precipitation ; 

 but, as a rule, freshly prepared solutions are the best. These should 

 be filtered before use. We may place a few drops of the filtered 

 solution upon the dried film on the slide or cover glass, or the thin 

 cover may be floated upon a little of the solution in a watch glass. 

 In some cases it is best to use heat to expedite the staining, and this 

 ma}' be done by holding the slide or the watch glass over the flame 

 of an alcohol lamp until steam commences to be given off. If the 

 heating is carried too far the preparation is likely to be spoiled by 

 the precipitation of the staining agent. As a rule, heating will not 

 be necessary, and when an aqueous solution of f uchsin (one part to 



