68 PRACTICAL BACTERIOLOGY 



the bouillon, and when the inoculation is completed, heat the side 

 bulb gently to melt the paraffin, so as to allow it, by slightly inclining 

 the flask, to pour on the film already formed above the bouillon. 

 Upon hardening, this additional paraffin forms an almost air-tight 

 layer, and when the flask is placed in the incubator, the heat causes 

 the paraffin to be pressed upwards into the constricted neck of the 

 flask ; this closure is further completed by the gases generated in the 

 culture. 



CXXVIII. THE INCUBATOR. 



Certain forms of bacteria develop at a higher temperature than 

 others. Pathogenic or disease-producing organisms grow more 

 luxuriantly at 37'5 C. than at lower temperatures, whereas the 

 ordinary saprophytic forms develop almost at any temperature 

 between 18 C. and 37'5 C. For the cultivation of pathogenic 

 bacteria a special apparatus is used, known as a thermostat or 

 incubator, made throughout of copper, with double walls, the space 

 between being filled with water. It is provided with a closely- 

 fitting double door with an inner door of glass, enabling the cultures, 

 etc., in the chamber to be inspected without actually opening it. The 

 regulation of the temperature within the incubator is maintained by 

 an automatic regulator of sufficient delicacy to prevent a fluctuation 

 of more than 0'2 C. in the temperature of the air within the chamber 

 of the apparatus. A Koch's safety burner is generally used for 

 heating; the safety attachment automatically turning off the gas 

 supply, thus preventing accidents should the flame be extinguished 

 when no one is near. 



For illustration of a modern incubator and various attachments in 

 position, see Fig. 19, p. 69. 



CXXIX. SPECIAL REACTIONS PRODUCED BY 

 BACTERIA DURING THEIR GROWTH. 



The reactions produced by many species of bacteria in the course 

 of their development in culture media are of great differential value. 

 These changes in some instances are so great that they can be 

 detected by simple reagents, whilst in others they are so slight as to 

 require the most delicate test for their demonstration. Some bacteria 

 produce at one period of their life an alkaline and at another an acid 

 reaction. This is seen in cultures of Loffler\s bacillus of diphtheria. 



These reaction differences are best observed when a chemical 

 substance, which does not interfere with the development of the 



