PRACTICAL BACTERIOLOGY. 



close up the test-tubes with, the greatest care. The 

 cotton-wool plugs should be taken out with strongly 

 heated forceps, held in a flame, and then rapidly 

 re-inserted in the test-tubes. In this manner the 

 outer impregnated portions of the plug are burnt off, 

 and the spores, which had fallen on it since the test- 

 tube was sterilised, are killed. It is also a good plan 

 to heat the upper part of the test-tube strongly in a 

 flame. In all cases the india-rubber cap must have 

 lain for half a day in corrosive sublimate solution 

 (1:1000). 



In conclusion, it may be mentioned that in a great 

 many cases a much simpler apparatus than the incuba- 

 tor may be used. The student can, at small cost, 

 construct it for himself, only, of course, it would not 

 be suitable for very accurate scientific experiments. 

 A strong plate of zinc, of the necessary size, is placed 

 on a strong wire tripod, and upon it a box made of 

 strong cardboard is put, the size and get up of which 

 may be varied according to taste. The most con- 

 venient size I found to be 30 cubic cm., outside 

 measurement. A second cardboard box is placed in 

 this one, smaller by 5 cm. in every direction than the 

 first, and between the two a thick layer of cotton-wool 

 is inserted. The boxes must lie closely upon the sheet 

 of zinc. A hole is made, into which a thermometer 

 fits as tightly as possible, an,d all chinks are then care- 

 fully stopped up with cotton-wool. A small paraffin 



