18 
Growth of Bacilli 
the thickness of the agar layer in the dish. This block has a smooth 
under and upper surface. Place it, under side down, on a slide and 
protect it from the dust. Prepare an emulsion in sterile water of the 
organism to be examined, if it has been grown on a solid medium, or use 
a broth culture ; spread the emulsion or broth upon the upper surface of 
the block as if making an ordinary cover-slip preparation. Place the slide 
and block in a 37° C. incubator for five to ten minutes to dry slightly. 
Then lay a clean sterile cover-slip on the inoculated surface of the block 
in close contact with it, usually avoiding air bubbles. Remove the slide 
from the lower surface of the block and invert the cover-slip so that the 
agar block is uppermost. With a platinum loop run a drop or two of 
melted agar along each side of the agar block, to fill the angles between 
the sides of the block and the cover-slip. This seal hardens at once, 
preventing the slipping of the block. Place the preparation in the 
incubator again for five to ten minutes to dry the agar seal. Invert this 
preparation over a moist chamber and seal the cover-slip in place with 
white wax or paraffin. The preparation may then be examined at 
leisure.” In most of Hill’s experiments a warm stage was made use of. 
The results obtained by Hill (1901) and by Hill and Rickards 
(1903), who appear to be the only workers who have investigated the 
growth of bacteria under such conditions, will be referred to later. 
Methods. 
1. Growth on the surface of agar. 
In order to investigate the mode of growth on the surface of agar 
the following very simple method was employed. A small quantity 
of melted agar was poured on to the surface of a sterile glass slide and 
immediately spread with a warm sterile platinum needle, so as to cover 
at least one square inch of surface. As soon as the agar had solidified 
a small drop of a very dilute emulsion of the organism to be investigated 
was placed on the surface of the agar, and a square three-quarter inch 
cover-glass was then gently lowered over the drop, avoiding as far as 
possible the formation of bubbles. The best plan of accomplishing 
this is to place one edge of the cover-glass near the edge of the agar 
and gently lower the opposite edge with a platinum needle. Next the 
agar projecting beyond the cover-glass was cut away on all sides 1 , and 
1 It was found that if the cover-glass was only sealed down by means of vaseline on to 
the surrounding agar (without removing the excess of agar) the preparation gradually 
contracted and the focus of the microscope had to be constantly altered. 
