FILM SPECIMENS 111 



of soda solution for a few minutes, well washing, and then 

 treating with strong sulphuric acid, warmed carefully in 

 a porcelain dish, for a few minutes. The acid having 

 been poured off, they are well rinsed in several changes of 

 water, and should be kept in a stoppered glass pot or 

 capsule in absolute alcohol. 



In order to prepare a film specimen, a droplet (i.e. small 

 drop) of tap-water or of physiological salt solution (not 

 distilled water) is deposited in the middle of a clean and 

 dry slide by means of a looped platinum needle, or with 

 a small glass pipette (Fig. 7). The slide may rest on the 

 table or upon a glazed tile. Theoretically, physiological 

 salt solution x sterilised by boiling should be used, but 

 ordinary tap-water may generally be employed. 



A mere trace of the growth is removed with a straight 

 platinum needle from the margin of the growth on the 

 surface of a young agar or gelatin or other solid cul- 

 ture medium, and this is well rubbed up with the 

 droplet of water on the slide, so as to form an emulsion, 

 which is then spread over the surface. As a general rule 

 the material should be well emulsified, but in some 

 instances this is inadvisable, as a particular formation or 

 characteristic grouping may be disturbed thereby, in 

 which case, after a slight admixture with the water, the 

 emulsion is gently spread. A minimum of growth should 

 be taken and the emulsion well spread, otherwise the film 

 will be too thick. The thin moist film is allowed to dry, 

 or may be dried by gentle warming over the Bunsen 

 flame, preferably holding the preparation in the fingers 

 and moving backwards and forwards above the flame. 

 The film, when dry, must next be fixed, which is accom- 

 plished by passing the slide, film side up, six times through 

 the Bunsen flame (a cover-glass is held in the forceps and 

 passed three times through the flame). Films may also be 



1 0-75-0-95 per cent, of sodium chloride dissolved in distilled water. 



