5 6 SOLID CUL TIVA DION MEDIA. 



3. Specific peculiarities in the mode of growth are more marked 

 than in the liquid media. In some solids {e.g., gelatine) the 

 naked eye appearances are in many cases absolutely diagnostic 

 of the presence of different species. 

 In the present chapter, the preparation of solid media, such as are 

 most frequently employed, will be considered, and each step in the 

 process will be described seriatim. The nutrient qualities of the soil 

 differ in the case of each solid used, some materials being specially 

 suited to the maintenance of one form of organism, others of another. 

 Sterility of the soil is insured by the employment of heat, the most 

 powerful sterilising agent at command. It may be attained either by 

 the application of a very great heat for a short time, or by prolonged 

 exposure to a much lower temperature. The action of steam at 

 ioo°C. is specially powerful in this direction; a comparatively short 

 period of exposure to this being sufficient to effect complete sterility. 

 As compared with dry heat at the same temperature, it is greatly 

 more potent, probably from the fact that in the hot air the germs 

 merely become perfectly dry, in which state they may be able to 

 resist a very high temperature. 1 When moistened by the aqueous 

 vapour, they are much more susceptible to the germicidal action of 

 heat, which in all probability produces coagulation of some element 

 essential to their life. The heat thus employed must be adapted in 

 its character to the nature of the material to be sterilised, substances 

 that might become too dry being steamed ; others, where partial 

 evaporation is desirable, being sterilised in a hot air chamber. 



A particular account of the modes of preparation follows, accom- 

 panied in each case by the methods of inoculation for artificial culture, 

 and the chief uses which each method of culture subserves. 



• 



{A.) Cultivation on Sterilised Potatoes. 



39. For this purpose medium-sized potatoes, oval in shape and 

 regular in contour, are selected. After a thorough washing in water, 

 any excrescences are removed, and a preliminary purification is effected 



1 According to Pasteur, the spores of Pencillium and other common mucedines 

 are not destroyed by a temperature of 120 to 125 C. in the absence of moisture (see 

 '■ Bacteria," Magnin and Sternberg, p. 171). 



