246 AIDS TO BACTERIOLOGY 



steam under pressure must have a higher disinfectant 

 value than steam not under pressure. 



Steam is used either confined under pressure or as a 

 current with or without a pressure exceeding that of the 

 atmosphere. The advantage of some amount of pressure 

 of saturated steam, however small, is that it gives a real 

 control over the temperature of steam, which in a well- 

 designed disinfector is practically uniform throughout. 

 It has also been repeatedly shown that in the absence of 

 pressure the temperature and disinfectant value of the 

 steam depend largely on its velocity, and the rate of 

 stoking will largely affect it an objection which is 

 serious because there is no convenient or trustworthy 

 means of controlling either the velocity of the steam or 

 the rate of stoking. What the temperature should be is 

 still a matter of discussion. Taking all the facts together, 

 the least exposure which can assure general disinfection 

 is fifteen minutes to air-free, saturated steam at 115 C. 

 This exposure may leave absolutely no margin of safety, 

 and the conditions of saturation, freedom from air, 

 temperature, and time of exposure must be rigidly assured 

 independently of the operation. 



Spray Disinfection. In spraying, the operator should 

 begin at the bottom of a wall and work upwards. If the 

 opposite is done, the deposited liquid runs in streams down 

 the dry wall, and staining results. Given an efficient 

 disinfectant, a sufficiently fine spray, and a conscientious 

 worker, spray disinfection of rooms is most satisfactory. 



For descriptions of sprays and steam disinfectors, see 

 'Applied Bacteriology.' For action of light, see p. 9; 

 for action of desiccation, see p. 8; for sterilisation by 

 filtration, see pp. 215, 239. 



Chemical Disinfectants. 



A disinfectant, or germicide, is a substance capable of 

 killing bacteria; an antiseptic inhibits bacterial growth; 

 and a deodorant prevents or absorbs foul smells. 



Theories of Chemical Disinfection. * Paul and Kronig 

 suggest that the degree of ionisation of a solution has an 

 important bearing on its disinfectant efficiency. It is 



* For further information on the theories of disinfection and on 

 the influence of the constitution of disinfectants on germicidal power, 

 see Professor Sommerville's ' Aids to Public Health.' 



