STERILISATION 9 



120 C. to 128 C. is necessary to attain the same result 

 with dry heat. While the spores of most pathogenic 

 bacteria are destroyed by boiling for a short time, those of 

 non-pathogenic species require a much longer exposure. 

 A temperature of 140 for three hours is necessary to 

 secure destruction of the spores of some organisms by dry 

 heat. B. subtilis spores are particularly resistant, and 

 Ellis found they resisted boiling at 100 C. for six hours 

 without injury to their germinating power. 



Fractional Sterilisation. Instead of performing sterili- 

 sation in one operation, ' discontinuous ' or ' fractional ' 

 sterilisation may be employed. For this the medium or 

 other material is exposed to heat for a few minutes on two 

 or three successive days. Any spores surviving the first 

 heating germinate, and the resulting organisms succumb to 

 the second and third sterilisations. Hewlett ascribes some 

 of the sterilising effect simply to the injurious action of 

 alternate heating and cooling. As spores may take days 

 or even weeks to germinate, the procedure is not always 

 certain in its action, and it is thus possible for spores to 

 germinate in a medium believed to be sterile. 



Light. Sunlight and, to a less degree, the electric arc 

 are very injurious to certain forms of bacteria. The red 

 and yellow rays of the spectrum have little effect, germi- 

 cidal action being exerted by the blue and violet portions. 

 The violet rays are therapeutically applied in the Finsen 

 light treatment of lupus vulgaris. Water and the ordinary 

 milk of commerce can be sterilised without appreciable 

 rise of temperature by exposure to the ultra-violet rays of 

 a quartz-mercury lamp. Hewlett and Barnard find that 

 these rays have practically no power of penetration, and 

 are stopped even by thin glass. The action of light is 

 superficial, even a short depth of water stopping the action. 

 Probably sunlight does not materially assist the purifica- 

 tion of rivers. Definite germicidal action cannot yet be 

 attributed to the Rontgen rays, while radium emanations 

 require long exposure and close contact to exert appreciable 

 action. 



Pressure. If suddenly applied or released, pressure 

 may rupture bacterial cells; otherwise no appreciable effect 

 has been observed. 



Electricity. The products of electrolysis may destroy 

 bacteria, and currents of high potential may inhibit 



