THE METHODS OF CULTIVATING MICROBES 65 



melted nutrient gelatine is now poured on the 

 surface of the agar-agar, and when it is cooled a 

 drop of a bouillon cultivation of Bacillus siibtilis is 

 run on to the surface from a capillary pipette. The 

 tube is then sealed, or the cotton-wool plug is 

 rendered impervious by being luted with warm 

 paraffin- wax. The object of growing Bacillus subtilis 

 is that it uses up the oxygen at the surface ; con- 

 sequently the microbe below receives none, or, in 

 other words, it is able to grow anaerobically. To 

 obtain inoculating material from Roux's tube, it is 

 broken at the bottom and a sterilised needle inserted 

 into the lower growth. 



FIG. 25. DROP-CULTURE CELL. 

 (With arrangement for admitting Gases into the Cell) 



In place of the Bacillus subtilis, the layer of 

 nutrient gelatine is covered with a solution contain- 

 ing one part of pyrogallic acid to ten parts of a 

 solution of potassium hydroxide (10 per cent.). The 

 potash solution of pyrogallic acid may be replaced, 

 with advantage, by a 3 per cent, solution of ferrous 

 sulphate, or a 2 per cent, solution of cuprous 

 chloride ; both of these compounds (the author has 

 found) prevent the entrance of air. 



A drop culture forms a useful method for study- 

 ing the growth and multiplication of microbes under 

 low or high power objectives. For this purpose a 

 glass cell is required. This is made by cementing 



