CULTURES IN LIQUID MEDIA. 65 
of culture fluid, attached to a thin glass cover and suspended over a 
circular excavation ground out of a glass slide, is very useful. 
Such a drop culture may be left under the microscope and kept 
under observation for hours or days. 
In making these drop cultures it is necessary to sterilize the glass 
slides and thin glass covers by heat, and to take every precaution to 
prevent the inoculation of the drop of culture liquid with any other 
bacteria than those which are to be studied. 
The simplest form of moist chamber for drop cultures consists of 
an ordinary glass slide having a concave depression, about fifteen 
millimetres in diameter, ground out in its centre. This and the thin 
glass cover, having been sterilized by exposure in the hot-air oven at 
150° C. for an hour or more, or by passing them through the flame 
of an alcohol lamp, are ready for use. The cover glass is held in 
sterile forceps, and a little drop of the culture fluid containing the 
bacterium to be studied is transferred to its centre by means of the 
platinum loop heretofore described. It is best to spread the drop 
out as thin as possible, and it may be inoculated, from a pure cul- 
Fie. 36. 
ture, with a platinum needle (Fig. 36) after it has been placed upon 
the cover. This is then inverted over the hollow place in the glass 
slide, and it is customary to prevent the entrance of air and attach 
the cover by spreading a little vaseline around the margin of the 
excavation. 
Another form of moist chamber is made by attaching a glass 
ring, having parallel, ground surfaces, to the centre of a glass slide 
by a suitable cement. 
In Ranvier’s moist chamber there is a central eminence sur- 
rounded by a groove ground into the glass slide, and the drop of 
culture fluid is in contact with a polished glass surface below as well 
as above. This affords a more satisfactory view under the micro- 
scope. 
The Author’s Culture Method.—In a paper read at the meeting 
of the American Association for the Advancement of Science, in 
August, 1881, the writer described a method of conducting culture 
experiments which he has since used extensively and with very satis- 
factory results. The liquid culture medium is preserved in little flasks 
having a long neck which is hermetically sealed. The principal ad- 
vantages connected with the use of these little flasks, or ‘‘ Stern- 
5 
