METHODS OF STUDYING BACTERIA 27 



Higher temperatures may be obtained by using boiling oils 

 and other fluids, whilst lower temperatures may be got by 

 'means of water-baths or special thermostat-ovens with various 

 automatic heat-regulating mechanisms. Vaccines and sera 

 have often to be sterilised at temperatures much below the 

 boiling point, in order not to destroy their potency. Most 

 non-sporing organisms are killed by prolonged heating at or 

 about 60 0., and this method is often made use of, as in the 

 "pasteurising" of milk and other fluids, in which organisms 

 such as Bacillus coli, Bacillus tuberculosis, etc., may be so 

 weakened or killed by an exposure of, say, twenty minutes 

 at 60 0. ( = 140 F.) as to be rendered innocuous. 



Exposure of infective material to the action of direct sun- 

 light, or in some cases even bright diffuse daylight, may kill 

 off the organisms present, whilst 

 the action of X-rays, Finsen light, 

 ultra-violet rays and other forms 

 of radio-activity may also be em- 

 ployed, especially as a therapeutic 

 measure, e.g. in ringworm, lupus (a 

 form of skin tuberculosis), favus, 

 certain forms of eczema, etc. 

 Ultra-violet light is sometimes used 

 for the sterilisation of drinking- 

 water, and specially devised appara- 

 tus for this purpose often forms 

 part of the modern military cam- 

 panning equipment. The sterilisa- 

 tion of milk and other fluids by 

 means of the electric current is 

 now being experimentally tested, and will in all probability bo 

 extensively used in the near future. 



Many fluids may be rendered sterile by the application of 

 heat at varying temperatures and for varying periods ; but 

 where any of their properties are destroyed by such a pro- 

 cedure, they may be passed through a suitable filter (see Figs. 6 

 and 7), itself previously sterilised, the pores of which are so 

 minute as to prevent the passage of bacteria. A fine grade of 

 unglazed porcelain or some similar material is usually employed 

 for this purpose. Various domestic filters of this description 

 are employed for rendering drinking-water germ-free (Fig. 6), 

 and fluids such as serum or fluid culture-media may also be so 

 treated. It is common knowledge that filtration through gravel 

 and sand is employed on a large scale for the purification of 

 water-supplies, though such filtration, from a bacteriological 

 point of view, is always incomplete, a certain number of micro- 



