80 APPLIED BACTERIOLOGY 



In addition to the various means described above of 

 keeping cultures out of contact with oxygen, various 

 additions of deoxydizing substances are added to the 

 culture medium, as used in ' stab ' cultures. The chief 

 substance used for this purpose is glucose (grape-sugar), 

 which, when added to the extent of 2 per cent, to nutrient 

 gelatine or agar, gives very good results in the case of the 

 bacillus of tetanus. Small additions of resorcine, formic 

 acid, and sulphindigotate of sodium have also been em- 

 ployed as additions to culture media for the purpose of 

 extracting oxygen. 



Hanging-drop Cultures. — With a platinum loop, a drop 

 of sterile broth is placed on a clean cover-glass which has 

 been passed through a flame. This drop is then inoculated 

 with a very minute trace of the organism under examina- 

 tion. A ' hollow ' slide is then taken — that is, one with a 

 concave excavation ground in the centre. The outside of 

 the well is then painted round with a narrow ring of 

 vaseline by means of a camel-hair brush ; the cover-glass, 

 with the drop of broth, is inverted and laid on the pre- 

 pared slide, and gently pressed down in such a way as to 

 make the cover- glass adhere firmly to the glass slide, so as 

 to make an air-tight joint, and thus prevent the drying up 

 of the drop by evaporation. 



The slide is then examined under the microscope — first 

 with a low power to find the edge of the drop, and then 

 with the higher powers, since, as it appears bounded by a 

 sharp line, the organisms in the drop can be more sharply 

 foeussed. The narrowest possible aperture of the diaphragm 

 must be used. This method of examination is exceedingly 

 useful in the examination of bacteria in their fresh state, 

 the progress and changes in growth and peculiarities, such 

 as motility, mode of propagation, and so on, being most 

 distinctly brought under observation. 



