CULTURES IN LIQUID MEDIA. 63 



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 ordinaiy 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- 



FIG. 86. 



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- 



