THERMAL DEATH-POINT. 513 



are used the cotton plug should be pushed down into the 

 tube (1-1.5 cm.) by means of sterile forceps. It should not 

 touch the rubber cap or cork. 



A very cheap and satisfactory way of sealing- tubes is 

 to employ sealing-wax. The cotton plug should be firm 

 and solid, and should not be pushed below the level of the 

 mouth of the tube. The heated wax should be first applied 

 to the edge of the cooled tube, and finally the center of the 

 plug should be covered. 



The tube cultures are usually kept in tumblers in a 

 darkened case. The author employs for this purpose black, 

 jacketed boxes shown in Fig. 73. The box is 6.5 cm. wide, 

 10 cm. deep and 18 cm. high. The lid is 9 cm. high, whereas 

 the inner height of the lower portion is 12.5 cm. A large 

 number of boxes can thus be arranged like books on a shelf. 

 The cultures are protected perfectly from dust and light. 



Thermal Death-point. 



\ 



As indicated heretofore, the vegetative and spore form 

 of the same organism differ in their resistance to destruc- 

 tion. Thus, while the former is readily destroyed by a tem- 

 perature of 60-70, the latter requires an exposure to steam 

 for some minutes. The vegetating forms of different species 

 of bacteria likewise show different degrees of resistance. 

 Some are killed readily at 55 and others require 65 to 70. 

 Moreover, it should be borne in mind that several varieties 

 of a given species may exist no two of which necessarily are 

 equally resistant. Thus, we may have spores of anthrax 

 which will be destroyed by a 5 per cent, solution of carbolic 

 acid within 24 hours, while another yield of spores from a 

 different variety of the same organism may resist this same 

 disinfectant for 50 days or longer. 



In studying- the resistance of an organism it is essential to pre- 

 pare as nearly a homogeneous suspension as possible. That is to say,. 



